Final answer:
Calvin needs to bike 25 yards to get back to the bottom.
Step-by-step explanation:
To find the number of yards Calvin needs to bike to get back to the bottom, we need to subtract the distance he ran up the hill from the distance he biked down the hill.
Given that Calvin ran up the hill for 100 yards and biked down the hill for 75 yards, the number of yards he needs to bike to get back to the bottom is:
100 - 75 = 25 yards