Question

three-sevenths of a number is 21. Find the number

Answer 1

Let x = the unknown number.
Because three-sevenths of the number is 21, therefore
(3/7)x = 21

That is
(3x)/7 = 21

Multiply each side by 7.
3x = 21*7 = 147

Divide each side by 3.
x = 147/3 = 49

Answer:  49