instruction:
The task is to generate the correct answer for a given math problem.
question:
Problem: a solution of 70 litres contains oil and water in the ratio 9 : x . if 6 litres of water is added to the solution , the ratio becomes 6 : 4 , find the value of x ?
answer:

question:
Problem: how many numbers between 400 and 1000 can be made with the digits 2 , 3,4 , 5,6 and 0 ?
answer:
60

question:
Problem: at a certain restaurant , the average ( arithmetic mean ) number of customers served for the past x days was 85 . if the restaurant serves 120 customers today , raising the average to 90 customers per day , what is the value of x ?
answer:
6