Rachel had to complete 8 pages of math homework, 7 pages of reading homework and 3 more pages of biology homework. How many pages of math and biology homework did she have to complete?
Let's denote the number of pages of math homework as $x$, the number of pages of reading homework as $y$, and the number of pages of biology homework as $z$. 

We know that:
$$x + y + z = 18$$
$$x = 8$$
$$y = 7$$

Therefore, we can calculate $z$:
$$z = 18 - 8 - 7 = 3$$

So, Rachel had to complete 8 pages of math homework, 7 pages of reading homework and 3 more pages of biology homework.