Question

Multiply $9999$ by $999$

Answer 1

$9999\times 999$

$=(10,000-1)\times (1000-1)$

$=10000(1000-1)-(1000-1)$

$=1,00,00,000-10,000-1000+1$

$=1,00,00,001-11,000$

$=99,89,001‬$