Answer:
Heidi wants to compute:
17 - 9.
First, she adds to get 10, (we want to subtract 10, this is easier than subtracting 9) then she can do:
17 - 9 + 0
adding zero does not change the sum
1 - 1 = 0
then:
17 - 9 + 0 = 17 - 9 + 1 - 1
Now we can do:
17 - 9 - 1 + 1
17 - (9 + 1) + 1
17 - 10 + 1
(17 - 10) + 1
7 + 1 = 8
Then, first she needs to add -1, in order to subtract 10, and then she must add 1. As we added a number and its inverse, in total we added zero, so the sum did not change, but was easier to compute.
Then, you need
