The task is to generate the correct answer for a given math problem.

Problem: 13 buckets of water fill a tank when the capacity of each bucket is 42 litres . how many buckets will be needed to fill the same tank , if the capacity of each bucket is 6 litres ?
91

Problem: which number can we add to 425897 to get a number exactly divisible by 456 ?
7

Problem: three pipes , a , b , & c are attached to a tank . a & b can fill it in 20 & 30 minutes respectively while c can empty it in 20 minutes . if a , b & c are kept open successively for 1.5 minute each , how soon will the tank be filled ?

2 hours 15 min