My puppy, Milkshake, dug 20 holes in the yard. ⅘ of the holes were in the front yard. The rest were out back. How many holes had Astro dug in the back yard?

20 holes total.

4/5 were in the front so that is 4/5 x 20 = 16 in the front.
Then in the back we have 20-16 = 4 holes.
OR, 4/5 in the front so 1/5 in the back and 1/5 x 20 = 4 holes in the back.