Sheri's age in 20 years will be the same as Jerry's age right now. Ten years from now, Jerry's age will be twice Sheri's. What are their present ages?
S+20=J
J+10=2(S+10)
S-J=-20
-2S+J=10
add equations
-S=-10
S=10
J=30
J+10=2(S+10)
S-J=-20
-2S+J=10
add equations
-S=-10
S=10
J=30