Question

If we set c=19!, then express 21!-20! in terms of c

Answer 1

`\begin{array}ll 21!\implies &21\cdot 20\cdot 19\cdot 18\cdot 17.....\\&\\ &21\cdot 20\cdot 19! \end{array} \begin{array}{llll} 20!\implies &20\cdot 19\cdot 18\cdot 17\cdot 16\cdot 15.....\\\\ & 20\cdot 19! \end{array} \\\\[-0.35em] ~\dotfill\\\\ (21\cdot 20\cdot 19!)-(20\cdot 19!)\implies (21\cdot 20\cdot c)-(20\cdot c) \\\\\\ 420c-20c\implies \text{\LARGE 400c}`