Mathematics(%age) Problems!?

[Professor Hockensie]

Please answer those difficult questions:- (Use my way)

1) When John Short increases the speed at which he motors from an average of 40mph and 50mph, the number of miles travelled per gallon decreases by 25%. If he travels 36 miles on each gallon when his average speed is 40mph, how many miles per gallon can he expect at an average speed of 50mph ?

2) When petrol was 50p per litre I used 700 litres in a year. The price rose by 12% so I reduced my yearly usage by 12%. Find:-

a) the new price of a litre of petrol

b) my reduced annual petrol usage

c) how much more(or less) my petrol bill is for this year

Example how to solve:

Decrease 450 Kg by 15%

Answer: Multiplying factor= 100-15/100= 85/100
Final answer= 85/100 * 450 = 765/2= 382.5 KG

Thank You so much !
And explain !