For word questions, you label your unknowns and then use substitution with one of your equations. For a system of more than two linear equations you can use a matrix or use substitution multiple times.
For your example,
Let l = time of the low speed pump to fill the pool, h = time of the high speed pump to fill the pool.
Assuming that the relationship is linear when using both the low speed pump and the high speed pump at the same time we get
l = h+3
h+l = 8