Answer:
-11/2
Step-by-step explanation:
Put the numbers into the expression and do the arithmetic.

Total number of sheets are 12.
Total number of white sheets= 9
Total number of red sheets = 3
Probability = Number of favorable outcomes/ total number of outcomes.
Hence
When Calvin throws the sheets in the air, the probability that he gets a white sheet is

which equals to 
that becomes 0.75
$7 because all you have to do is find the greatest common denominator. Which is 7. 7 is the greatest common denominator in the bunch. 7 goes into 21, 42, 49.