Question 13175
You know that the entire class is 100%, because 100% is the whole thing.  So, figure out what percents you know already (10% had A's, 25% had B's, 30% had C's, and 15% had D's).  Add those percentages up to see what you know (10+25+30+15 = 80%).  You know now that 20% of the class failed the test.

Now you need to figure out what 20% of 80 is.  We are doing 20% of 80 because there are 80 people in the class.  To solve this, change 20% to a decimal (.20) and multiply by 80 (we are multiplying because "of" means to multiply in math). So, your equation looks like:  {{{.20*80 = 16}}}.  So, 16 people failed the test.