Pamela's age is three times Jiri's age. The sum of their ages is 28 . What is Jiri's age?

So, if we say Jiri's age is j (we can pick any letter), then Pamela's age is 2j, right?

j + 2j = 72

j is the same as 1j, but we wouldn't call you 1James, just James.

3j = 72

To isolate j, we do the opposite operation to both sides.

3j/3 = 72/3

j/j = 1, so that cancels out. 72/3 = 24.

j = 24

Jiri is 24 years old.