A patient is supposed to take 10 mg. of a certain drug. If he already took 7 and 5/8mg., how many more milligrams does he have to take?

First, convert 7 and 5/8 into an improper fraction: 61/8. Then, you convert 10 into an improper fraction: 10/1. Next, find the least common denominator, which is 8, and multiply 10/1•8/8=80/8. Then, subtract 80/8-61/8=19/8. 19/8 cannot be simplified farther, but if you want, the mixed number would be 2 and 3/8 more mg that the patient has to take.