The reasoning: Each brother can choose 5 friends from 15 in ways. So the number of ways two brothers can choose is .
numerator = number of ways the two brothers can choose 6 different friends: one brother chooses 5 friends from the 15, then the other brother chooses 4 friends from that 5 and 1 friend from the remaining 10.
denominator = number of ways the two brothers can choose 5 friends each = answer from part (a).
But since arrangements under restriction aren't my forte, you should probably wait for confirmation or refutation ......