a) What must 2 1/7 be divided by to give 1 3/7 ?

b) What must 3 4/5 be multiplied by to give 10 2/5?

It's a pre-algebra, no calculator allowed question.

Do i have to just keep guessing and substituting guesses until i get the answers? Or are there exact formulas for solving these problems? (Youcantuse an algebra formula because its from a pre-algebra book and algebra has not even been covered yet)