I also got ans C, but my way of thinking was following-

we have 10 A's and 15 B's , so 150 variations and N!/2!(N-2)!

a)N!/2!(N-2)! =12!/2!*10!=66

b)105

c)136

d)171 (above 150, so exclude D and E)

since C is the greatest , then answ C

I agree that my way of thinking is more complicated than yours, but please let me know if it was a right one )) if not, where was I wrong?

p.s. I got my answ before looking at the right answ

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