SOLUTION
a. The number of possible sums, when at least one must be selected becomes
[tex]\begin{gathered} \text{Selecting from 4 bills is 1 way} \\ Selecting3from4becomes^4C_3=4\text{ } \\ \text{Selecting 2 from 4 becomes }^4C_2=6 \\ \text{Selecting 1 from 4 becomes }^4C_1=4 \\ \text{Total possible sums = 1+4+6+4 = 15} \end{gathered}[/tex]Total possible sums for a becomes 15
b. The possibility of the student not selecting at least one means he will select nothing and this leaves us with 15. So the answer to b is 15
