Consider the coefficient of in . Each of the three factors corresponds to an envelope. The power of coming from the first, second, or third factor and contributing to the total power of 11 corresponds to the number of coins in the first, second, or third envelope. That's why the minimum power of in each factor is 2. The maximum power is 7 because each envelope has at most 7 coins (7 + 2 + 2 = 11). One can select, say, from the first factor, from the second and from the third; this would be different from from the first factor, from the second and from the third. Both of these variants contribute 1 to the final coefficient of . This means that the envelopes are distinct.

Can you now solve the second problem?