When one quarter of a number is subtracted from one third of the number, the result is 7

The number ... n
one quarter of the number ... 1/4 of n = 1/4 * n
one third of the number ... 1/3 of n = 1/3 * n
one quarter of the number is subtracted from one third of the number ... 1/3 * n - 1/4 * n
the result is 7 ... = 7

We can put it together: 
1/3 * n - 1/4 * n = 7
4/12 * n - 3/12 * n = 7
1/12 * n = 7     /*12
n = 7 * 12
n = 84

The number you are looking for is 84.