Kareen correctly answered 85% of the questions on his last math test. If he missed 6 problems, how many questions were on the test?

He got 85% correct, which also means he got 15% wrong (the 6 problems)

This means 6=15%

15 does not go into 100 evenly, but 5 does. And 3 happens to be a common factor for both 6 and 15. If you divide both sides by 3 you get:

2=5%

Now we need to find 100%. If 5% of the test is 2 questions, then 100% must be 40.

It's like solving proportions-

2:5

x:100

5 to 100: multiply by 20

2 to x: also multiply by 20

2 times 20 = 40 = 100%

The total number of questions on the test is 40 (questions).