Stretch y thinking

Question

When you are subtracting from a 3 digit number, how do you know if you will need to ungroup?

Answer 1

342- 157 = 185

7>2 so borrow 1 from the 4 to get 12, then subtract 7 from 12 to get 5 as the ones' digit in the answer

that leaves 33 - 15. it would have been 34-15, but you borrowed 1 from the 4, so it's 33 -15, still 3 is less than 5, so borrow 1 from the other 3 to get 13-5 = 8 for the tens' digit

now that leaves 2-1 = 1 for the hundreds' digit

that leaves 185 as the answer

or better yet use a calculator