57. If $a, b, c$ are all multiples of 5, $a<b<c, c=a+10$, then $\frac{(a-b)(a-c)}{b-c}=$
Answer: -10.
Solution: From the given conditions, we get
$$
b=a+5,
$$

Therefore,
$$
a-b=-5, a-c=-10, \quad b-c=-5 \text {, }
$$

Thus,
$$
\text { the original expression }=\frac{(-5) \times(-10)}{-5}=-10 \text {. }
$$