Question

it takes Joey one-sixteenth of an hour to write one thank you card how many cards can he write in three fourth of an hour

Answer 1

You want the two fractions of an hour's denominators to match. So we have to convert three-fourths of an hour to sixteenths:

3/4 * 4/4 = 12/16

So, since he's writing for twelve sixteenths of an hour,  you multiply how many he can write in one hour by twelve:

1 card per 1/16 of an hour * 12 sixteenths of an hour =
12 cards per 12/16 hour