The coefficient of x¹⁸ in the product
(1 + x) (1 – x)¹⁰ (1 + x + x²)⁹ is :

The smallest natural number n, such that the coefficient of x in the expansion of $${\left( {{x^2} + {1 \over {{x^3}}}} \right)^n}$$ is ⁿC₂₃, is :

If the coefficients of x² and x³ are both zero, in the expansion of the expression (1 + ax + bx² ) (1 – 3x)¹⁵ in powers of x, then the ordered pair (a,b) is equal to :

If some three consecutive in the binomial expansion of (x + 1)ⁿ is powers of x are in the ratio 2 : 15 : 70, then the average of these three coefficient is :-