Pamela is 13 years older than jiri. the sum of their ages is 53 what is jiri's age?

You are first told that Pamela is 13 years older than Jiri so:

p=j+13

Then you are told that the sum of their ages is 53 so:

p+j=53

We already know from the first statement that p=j+13, this makes p+j=53 become:

(j+13)+j=53  so we combine the two j's on the left side

2j+13=53  subtract 13 from both sides

2j=40  divide both sides by 2

j=20

So Jiri is 20 years old.