Anumber is one seventh of another number the difference of the number is 18 assume u are subtracting the smaller number from the other number

Let the first number be x and the second number be y.  Then x-y = 18, assuming that x>y.  y, the smaller number, is 1/7 th of x.

x-y=18 and y=x/7 govern this situation.

Let's find x.  Subst. x/7 for y in x-y=18:  x-x/7 = 18

Mult. all 3 terms by 7 to remove fractions:  7x - x = 126

Combine:  6x = 126; solve for x by div. both sides by 6:     x = 21

The larger number is 21 and the smaller number is (21-18) = 3.