You can set equations here;
C stands for Clare
R stands for Ryan
J stands for Jerry
C + R + J = 270
C = 25 + R
J = 19 + C
So our goal is to solve for J.
Start by isolating R; we can do that on second equation.
C - 25 = R
Now plug that in first equation;
C + (C-25) + J = 270
Now we need to isolating C; we can do that on third equation;
J-19 = C
Plug that in equation we recently work on;
(J-19) + ( (J-19)-25 ) + J = 270
Simplifying;
(J-19) + (J-44) + J = 270
J+J+J - 19 - 44 = 270
3J - 63 = 270
Now solve for J;
3J = 333
J = 111
So Jerry collected 111 used books.