Question

Sunny elementary school is having its annual read-a-thon.the third graders have read 573 books so far. their goal is to read more than 900 books.

what is the least number of books they need to read to reach their goal

Answer 1

So first you take the number already read, in this case 573 books.
Next you take the goal, 900

Do the Goal minus the amount done:
900-573=327