On a plan scale where 1/8 of an inch equals 1 foot, how long would 2.5 inches represent?

To determine how long 2.5 inches represents on a plan scale where 1/8 of an inch equals 1 foot, you start by establishing the relationship between the inches on the scale and the actual feet.

Since 1/8 of an inch is equivalent to 1 foot, you can first find out how many 1/8 inch segments are in 2.5 inches. This is done by converting 2.5 inches into eighths:

2.5 inches is the same as (2.5 \div \frac{1}{8} = 2.5 \times 8).

Calculating that gives you:

(2.5 \times 8 = 20) eighths of an inch.

Since each of these eighths corresponds to 1 foot, 20 eighths of an inch would represent 20 feet in real life.

Thus, the correct answer is that 2.5 inches on the plan scale equals 20 feet, aligning with choice C.