The coefficient of a³b⁴c⁵ in the expansion of (bc + ca + ab)⁶ is

If c₀, c₁, c₂, ......, c₁₅ are the binomial coefficients in the expansion

of (1 + x)¹⁵, then the value of $${{{c_1}} \over {{c_0}}} + 2{{{c_2}} \over {{c_1}}} + 3{{{c_3}} \over {{c_2}}} + ... + 15{{{c_{15}}} \over {{c_{14}}}}$$ is

7²ⁿ + 16n $$-$$1 (n$$ \in $$ N) is divisible by

The number of irrational terms in the expansion of $${\left( {{3^{{1 \over 8}}} + {5^{{1 \over 4}}}} \right)^{84}}$$ is