I'd think that in a) it's rather n=9 and r=4...

The author was right.

 I have made mistakes in the computations of answers to question (a) and (b).

Corrected answer:(a) [math]\binom{n + r -1}{r}= \binom{12}{9}= 220 [/math] where n=9 and r = 4 

Corrected answer:(b) [math]220 + \binom{15}{13} + \binom{14}{12} + 2 \times \binom{13}{11} =220 +105 +91 + 156 =572[/math]

 

Hence, author's answers are correct. 

Edited March 3, 20223 yr by Dhamnekar Win,odd
edited latex to work