(a) In the binomial expansion of \(\left(a-\dfrac{x}{2}\right)^6\), the coefficient of \(x^3\) is 120 times the coefficient of \(x^5\). Find the possible values of the constant \(a\).

(b) (i) Expand \((1+2x)^{20}\) in ascending powers of \(x\), as far as the term in \(x^3\). Simplify each term.

(ii) Use your expansion to show that the value of \(0.98^{20}\) is \(0.67\) to 2 decimal places.