Question 12
Find the units digit of A, where
$A=\underbrace{20\times20\times\cdots\times20}_{100}\times\underbrace{15\times15\times\cdots\times15}_{100}$.
The braces show 100 factors of 20 and 100 factors of 15.
$A=\underbrace{20\times20\times\cdots\times20}_{100}\times\underbrace{15\times15\times\cdots\times15}_{100}$.
The braces show 100 factors of 20 and 100 factors of 15.