Answer:
a = 15
Step-by-step explanation:
The Pythagorean theorem applies:
a² + 20² = 25²
a² = 625 - 400 . . . . . subtract 20²
a = √225 . . . . . . . . . . take the square root
a = 15
_____
You might recognize that the given measures (20, 25) are in the ratio 4:5. There is only one right triangle with sides in that ratio: the 3:4:5 right triangle. That is, the unknown side (a) matches
a : 20 : 25 = 3 : 4 : 5 . . . . numbers on the left are 5 times those on the right
a = 5·3 = 15
