itopicman:
[color=#770077][/color]It's interesting to still see the interest of many people at attempting to solve questions on the greatest mental tasking subject -Mathematics. I must confess that the numerous solutions I've seen gladden my heart, although I do not agree with them. Patiently follow the solution.
1) This is a complex form of a Compound Interest question. (Reason: You need to keep on adding the monthly savings he makes per month.)
The mistake from many is to assume a one-time principal.
For clarity, at the end of the first year, the Principal = #2000 x 12 = #24000. 12% interest on this gives 24000 x 0.12 = 2880.
At the end of the 2nd year, the New Principal = (Principal of year 1) + (Savings/principal of year 2) + Interest from year 1 = 24000 +24000 +2880 = 50880. Interest from year 2 = 50880 x 0.12 = 6105.6. At the end of year 3, the New Principal = (year 1 principal) + (year 2 principal) + (year 3 principal) + (Interest from year 2) = 24000 + 24000 + 24000 + 6105.6 = 78105.6. and on and on like that.
As a mathematics student, I computed an analytical equation for this 'progressive compound interest' problem thus.

Amt=yp*(n+((n-1)*x)+((n-2)*x^2)+((n-3)*x^3)+((n-4)*x^4)+((n-5)*x^5)+((n-6)*x^6)+((n-7)*x^7)+((n-*x^+((n-9)*x^9)+((n-10)*x^10)+((n-11)*x^11)+((n-12)*x^12)+((n-13)*x^13)+((n-14)*x^14)+((n-15)*x^15)+((n-16)*x^16)+((n-17)*x^17)+((n-18)*x^18)+((n-19)*x^19))
yp = yearly savings/principal = #24000,
n = number of years = 20,
x = interest rate per annum = 12% = 0.12,
Amt = Amount at the end of 20 years
Amt = # 541,700.

OR

Amt = 24000 * [20 + 19(0.12) + 18(0.12)^2 + 17(0.12)^3 + 16(0.12)^4 + 15(0.12)^5 +14(0.12)^6 + 13(0.12)^7 + 12(0.12)^8 + 11(0.12)^9 + 10(0.12)^10 + 9(0.12)^11 + 8(0.12)^12 + 7(0.12)^13 + 6(0.12)^14 + 5(0.12)^15 + 4(0.12)^16 + 3(0.12)^17 + 2(0.12)^18 + (0.12)^19]
Amt = # 541,700.
I love Mathematics !

adeoflagos: According to the World Bank, the inventor is Alfredo Moser (use Google). For now, its application is limited to day-time in human settlements (e.g. slums) where rooms are in darkness day or night. I read someone (SayITnw) write something about seeking cheap publicity on nairaland for such a great and in-use concept. How much publicity does an idea that is already on the website of a global financial institution need. "Nada"!!! Not from nairaland.

How could this be more expensive than buying electric light bulbs (ELB)? You pay for the ELB as well as the electricity. How much energy goes into production of the ELB and the electricity, which are likely produced from non-renewable sources.
You seem to have forgotten that we don't pay for electricity just to light bulbs. How much kWatt does a bulb condume in a room housing an electic iron, electric stove, refrigerator, AC, TV sets, etc. ? less than 8%! If u caim that the bulb is expensive, is the bottle free? You have to buy it afterall. The dividing lines between this Mosar bulb and the "Legendary" Faraday's bulb are - reliability & efficiency. You can use the bulb at anytime & at steady brightness. The mosar bulb is dependent on an astronomical body - the sun, and is therefore time-dependent. It deems off towards the night.
More so, some energy that is converted from electric energy to light energy by some bulbs is lost as heat. The initial cost of the solar light bulb (SLB) will likely be recouped over the life time of its use. The other benefits (i) wastes bottles are recycled and reused
The usefulness of any idea (sometimes cheaply called innovation) is in weighing its merits against its demerits. Is the recycling process the drilling of rooftops? This will only pave way to imminent leakage from the roof. Again, how do use it even use it in nigeria when most buildings have ceilings below the roof? Do u drill the ceilings too or u produce some 5-feet bottles ?
(ii) in the long-term the amount of waste-bottles that end up in landfills is reduced and the energy cost for transportation of wastes-bottles to the dump/landfill is reduced
Well I think in the long run this isn't going to reduce waste-bottle dumping because it only looks practicable in "Slums" , whose inhabitants translate to what in global population? maybe some 0.0x% (iii) it empowers individuals installing the solar light bulbs to generate income (electric bulb makers are large corporation not individuals).
What is ur defination of "individuals" when millions of people not having a roof directly above them cannot use this? Milions of people living under storey buildings cannot use it.
Other indirect benefits include less green-house warming (think about the production processes used for electric light bulb production, the CO2 from electricity generation, and the CO2 released during transportation of wastes bottles to the landfill).
Are you sure about no production of CO2? What does the plant powering the machines fot producing the plastic bottles releasing to the atmosphere? Except the plastic bottles are natural too, just thinking!
Young Nigerians need to learn to think holistically, so that they do could better than their parents who successfully put the country in the mess that it is in now.

adeoflagos: [/quote]

adeoflagos:

[quote author=adeoflagos]

yunfazz: What a brilliant idea!

A Brazilian mechanic has invented a way of lighting bulbs without electricity- using plastic bottles filled up with water and a splash of bleach.

The secret: Adding two capfuls of bleach to a bottle of water stops it from turning green when exposed to sunlight. He then drills a hole in a roof tile and pushes the filled bottle in from below. He keeps the bottle in place with polyester resin, which waterproofs the 'window' in his roof. More photos after the cut:

During a power shortage, it was his boss that first suggested the use of a plastic bottle filled with water as a lens to focus the sun's rays on dry grass and to light fires in case of an emergency. It was after then that Mr. Moser developed the concept into his Moser Light.

In a statement to the BBC World Service, he was quoted as saying: 'It's a divine light. God gave the sun to everyone, and light is for everyone. Whoever wants it saves money. You can't get an electric shock from it, and it doesn't cost a penny."

Wow..!!! The man should patent his invention immediately and not forget to fly to Nigeria asap too...we have plenty of water bottles and sunlight!!!

http://www.tutubranch.com/2013/08/plastic-water-bottle-bulbs.html