240x3= 720
720+80 = 800
800/20= 40
The answer is 40.
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Answer:

Apply the formula for sum of cubes: 
Therefore:


Answer:
E. 36
Step-by-step explanation:
You have to do a² + b² = c²
Plug in
15² + b² = 39²
225 + b² = 1521 (subtract 225 on both sides)
√b² = √1296 (square root on both sides)
b = 36
Hope this helps ya!