If the rise of the ramp is 21 inches, how many risers are needed when each riser is no more than six inches high?

To determine the number of risers needed for a ramp with a total rise of 21 inches, when each riser is no more than six inches high, you divide the total rise by the maximum height of each riser.

In this case, dividing 21 inches by the maximum riser height of six inches yields:

21 inches / 6 inches = 3.5 risers.

Since you cannot have a fraction of a riser, you round up to the next whole number. Therefore, 4 risers are necessary to meet the requirement for the total rise of 21 inches, ensuring that the ramp is compliant with building codes that suggest riser heights should not exceed the specified limit.

These calculations establish that the chosen answer is correct, as four risers will adequately accommodate the rise while staying within the maximum height allowance.