Molly has a light fitting with 5 bulbs. She swaps old bulbs which consumed 25 watts of power per bulb for low energy bulbs which only consume 6 watts of power each but have the same brightness. The box of the new bulbs claim a ‘70% reduction in power when compared to 25 watt bulbs’.
a) Is the claim correct? Show all workings.
b) The old bulbs used to cost 30p per bulb per month to run. Assuming the same amount of usage, and the same cost of electricity, how much money will Molly save per year in future by swapping low energy bulbs in this light fitting?
This is a very practical video, where Maths is used to justify a decision. The new Maths GCSE is using ‘prove’ style questions increasingly. This percentage question takes most students less than 4 minute to solve, and as such, the extension questions often work well to stretch more able students.
Teachers could extend the learning by considering:
- Each bulb cost £7.35 to buy. Assuming the same cost saving per year in future, how many whole years will it take before the investment in the bulbs has been covered by the cost savings?
- When calculating future cost savings, it has been assumed that energy prices remained unchanged. Sadly energy costs are rising by an average 8% year on year (compound interest). What would be the overall saving to Molly (remembering the purchase price of the bulbs) after 5 years of use?