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.