Thomas correctly spell 18 out of 20 words on his last spelling quiz. What decimal represent the portion of the rights that Thomas spelled correctly

The answer is 0.90

I used a rather ghetto way to answer this question.

first, I multiplied 20 by 5 to get it to 100 then I multiplied 18 by the same number (5) and got 90. That makes 90 out of 100 which is 0.90