6 pennies, 12 nickels, 8 dimes, 4 quarters = 30 coins
a) add the probability for drawing a dime and drawing a quarter together:
12/30 + 4/30 = 16/30 = 8/15
The probability is 8/15 or 53%
b) multiply the probability for drawing a penny and drawing a nickel:
6/30 x 12/30 = 72/900 = 2/25
The probability is 2/25 or 8%
c) multiply the probability for drawing quarters but take away one of the total coins and the amount of quarters each time:
4/30 x 3/29 x 2/28 = 24/24360 = 1/1015
The probability is 1/1015 or 0.099%