Answer:
3/7
Step-by-step explanation:
The prime numbers that lie between 1 and 21 are 2,3,5,7,11,13,17, and 19. These 8 balls, plus the 10-ball comes to a total of 9 balls that could be picked out of the 21. Therefore, the probability of picking one of these balls is 9/21 or 3/7
7a + 6c + 9a - 15c
-- Look for all the 'a's 7a, 9a
-- Addum up 16a
-- Look for all the 'c's 6c, -15c
-- Addum up -9c
-- Write the results 16a - 9c
7 inches = 49 cents
8 inches = 56 cents
11 inches = 77 cents
Just invert it like:

There you have your common denominator! Don't forget to multiply the numerator as well.