Их / 5 = 21 / 15

To solve this equation, you would cross-multiply and then divide to find the value of "их".

First, cross-multiply:

5 * 21 = 15 * "их"

105 = 15 * "их"

Next, divide both sides of the equation by 15 to isolate "их":

105 / 15 = "их"

7 = "их"

Therefore, "их" equals 7.