Thanks for the full problem. You did part A correctly but wrote "large shirt" instead of "red shirt" in the denominator. You correctly used 0.35, the probability of a red shirt.
The formula for conditional probability applied here says:
P( large | red ) = P(large & red) / P(red)
We have n=100 shirts, 5 of which are large and red
P(large & red) = 5/100
P(red) = 35/100
P(large | red) = (5/100)/(35/100) = 5/35 = 1/7 = 0.14285...
Part A answer: 14.3%
P(small | red) = P(small & red) / P(red) = (30/100)/(35/100)=6/7=.8571...
Part B answer: 85.7%
