You can use estimation to check for reasonableness when you have math problems with all numbers. When you do use estimation, it won't tell you whether you actually have the correct answer, but it will tell you if you're close and your answer is probably right.
For problem 4? I don’t see where that is but how that can work is you get the closest POSSIBLE number, or the exact number for the answer through all the calculations and formulas you follow, and the rules for shapes and angles.
