a) A tap can supply eight gallons of gasoline daily to each of its 250 customers for 60 days. By how many gallons should each customer's daily supply be reduced so that it can supply 50 more customers for twenty more days?

Total supply: $$8\times250\times60=120000$$ There will be 300 customers for 80 days, so the number of gallon for each customer is $$\frac{120000}{300\times80}=5$$ Each customers will need to reduce 3 gallons, (each customers will have 5 gallons only).