A triangle has side lengths measuring 20cm, 5cm, and n cm. Which describes the possible values of n?

The sum of any two sides must be greater than the third side.

So if n is the largest side:

n<20+5

n<25

And if 20 is the largest side:

n+5>20 subtract 5 from both sides

n>15

Since n>15 and n<25

15
n = (15, 25)