This is a two part question: First it asks to expand $\displaystyle (a + b)^5$, and then to find the coefficient of $\displaystyle x^3$ in the expansion of $\displaystyle (1/4 + 2x)^5$.

I know for part one, you get $\displaystyle a^5 + 5a^4b + 10a^3b^2 + 10a^2b^3 + 5ab^4 + b^5$, and that somehow the values of the following are substituted in this answer, but I am not really sure on how this is done or written. Could someone please help in this?