Answer: $1.77
=======================================================
Step-by-step explanation:
Use of a calculator makes quick work of this type of problem
5.25 + 2.98 = 8.23
10.00 - 8.23 = 1.77
-----------
If you want to do this using mental math, then one trick is to do this:
Think of 2.98 as 3.00, then add this to 5.25 to get 8.25. Notice that we're 2 cents too high (because of the jump from 2.98 to 3.00), so we need to subtract 2 cents from 8.25 to get 8.23
So that's a fairly quick way to add those two numbers in your head. The more practice you get, the quicker it will happen.
As for the subtraction part, you have a number of routes. Possibly a quick way is to jump from 23 cents to 30 cents (plus 7), then jump from 30 cents to 100 cents (plus 70). Therefore, we have 7+70 = 77 as the overall jump to get us to the nearest dollar. But we'll need one more extra dollar to end up at $10 exactly.
In other words, the "plus 77 cents" portion only gets us from $8.23 to $9, so that's why we need that extra dollar in change. Therefore, we've computed $1.77
I apologize for being a bit wordy. Despite being a bit long-winded, this mental math should come fairly quickly after enough practice.