Edmund's Brainteasers
Problem 17
Solution
Problem 17: Saving heat. Solution
Suppose all three heat saving methods were employed. Fitting a new boiler makes the new bill 55% of what it would be otherwise, putting insulation in the roof makes the new bill 70% of what it would be otherwise, and repairing the window frames makes the new bill 75% of what it would be otherwise. Adopting all three heat saving methods means our new bill would be
55/100 x 70/100 x 75/100 = 0.28875
times the old bill. We would therefore save (1-0.28875) x 100 %, that is 71.125%.