Lois got 27 problems right and it was 90% how many problems did she have

Lois had a total of 30 problems

Here's why:

One of the many ways we can figure this out is by noticing we are looking at 27 being 90% of the number we're looking for. This can be re-worded to say 27 equals  of a number, since 90% = 9 tenths. Now we can ask ourselves, "What number is multiplied by 9 to equal 27" That would be the number 3. Now we multiply 3 x 10 = 30. This method may not always work though. 

The most mathematical, and easiest way to figure out this problem is to divide 27 ÷ .9 which will give us 30. To check this answer we can multiply 30 * .9 and we get 27, our original number. 

I hope this helped! Have an amazing day :))