v = 90 documents per hour is the rate

t = 2 1/2 hr = 2.5 hr is the time

d = the number of documents

Since the rate equals the number of documents divided by the time follows the number of documents equals the rate multiplied by the time:

v = d/t multiply both sides by t

d = vt

Plug the values for v and t into the equation above:

d = 90*2.5

d = 225 documents

In 2 1/2 hours you can file 225 documents.