Answer:
15 units
Step-by-step explanation:
By using the Pythagoras Theorem,
C^2 = A^2 + B^2
Substitute in the given values,
25^2 = a^2 + 20^2
625 = a^2 + 400
625 - 400 = a^2 + 400 - 400
a^2 = 225
= 15
Hope this too helped! ^^
I'd recommend watching a video or two on the topic of Pythagoras Theorem, if this explanation isn't clear enough.