A "sum" is the answer for an addition problem. Knowing how to add is an important math skill. For these problems you will need to find the missing number and then match another problem having the same missing number.
3+_=8 You decide the missing number must be 5. Your choices include: 4+_=6, but the answer is '2', so it is not a match. 5+_=9, but the answer is '4', so it is not a match. 1+_=6, and the answer is '5', so it is 'a match'.
7+_=10 You decide the missing number must be 3. Your choices include: 3+_=7, but the answer is '4', so it is not a match. 5+_=8, and the answer is '3' so it is 'a match'.
For these problems, find the missing number and then choose another problem whose answer is 'a match'. Strive for accuracy, then strive for increasing your speed. Good luck and enjoy the challenge!