I'm thinking like this: Lets divid total distance(-- = 12.5 miles and each digit = 25 miles) 0--|--1--|--2--|--3--|--4--|--5--|--6--|--7--|--8 1. He needs 2 gallons of water to travell 25 miles 2. He needs 8 gallons of water to travell 50 miles similarly, 32 for 75 miles (I'm not sure... ) If it works then our general formula will be 2(4 pow(n-1)) to reach at position 8 2(4 pow(8-1)) total distance he needs to travell to cover 200 miles and come back 2(4 pow(7)) X 25 miles total number of round trip (2(4 pow(7)) X 25)/200 =(4 pow(7))/4