Frank can make 3 bird houses in 12 days. How many days will it take him to make 21 bird houses?

5.25

Step-by-step explanation:

3 divided by 12 = 0.25 (this how many he can make per day)

Now that we have how many bird houses per day (0.25), we can multiply that for the amount it takes. (21 bird houses) to get 5.25