Question Number 180115 by universe last updated on 07/Nov/22
Answered by mr W last updated on 07/Nov/22
$$\mathrm{7}×\mathrm{6}×\mathrm{4}×\mathrm{5}×\mathrm{2}=\mathrm{1680} \\ $$$$=\mathrm{40}×\mathrm{42}=\left(\mathrm{42}−\mathrm{1}\right)\left(\mathrm{41}+\mathrm{1}\right)=\mathrm{41}^{\mathrm{2}} −\mathrm{1} \\ $$$$\Rightarrow\:\left({d}\right) \\ $$