Их / 5 = 21 / 15

To solve this equation, we can cross multiply.

Cross multiplying means multiplying the numerator of the first fraction with the denominator of the second fraction, and vice versa.

In this case, we have:

их * 15 = 5 * 21

Multiplying the numbers:

15их = 105

Now, we need to isolate их by dividing both sides of the equation by 15:

их = 105 / 15

Simplifying the division:

их = 7

Therefore, the value of их is 7.