131.
A box has 6 black, 4 red, 2 white and 3 blue shirts. What is probability of picking at least 1 red shirt in 4 shirts that are randomly picked?
Answer: C
There are \( 6 + 4 + 2 + 3 = 15 \) shirts, of which 11 are not red. Use the complement.
\[ P(\text{no red}) = \frac{{}^{11}C_{4}}{{}^{15}C_{4}} = \frac{330}{1365} \]
\[ P(\text{at least one red}) = 1 - \frac{330}{1365} = \frac{1035}{1365} = \frac{69}{91} \]
Counting "exactly one, exactly two, exactly three, exactly four" would need four separate terms; the complement needs one.
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