The coefficient of $x^5$ in the expansion of $(1+x)^{21}$ + $(1+x)^{22}$ + ...... + $(1+x)^{30}$ is

Question

$\begin{array}{1 1} =^{30}C_5 +^{20} C_5 \\ =^{51} C_5 \\ =^{31} C_6 -^{21} C_6 \\ =^{30}C_5 -^{20} C_5 \end{array}$