Pamela's age is two times Jiri's age. The sum of their ages is 87. What is Jiri's age?

Let P be Pamela's age

Let J be Jiri's age

P = 2J (Pamela is twice Jiri's age) (1)

P + J = 87 (combination of their ages is 87)

P = 87 - J (thus Pamela's age would be 87 minus Jiri's age) (2)

Plug (2) into (1) (replacing P with its value)

Rearrange equation to find J (Jiri's age)

87 - J = 2J

87 = 2J + J

87 = 3J

87/3 = 3J/3

29 = J

J = 29

Note that you can do several more steps to check the answer and/or find Pamela's age.