The sales tax on a $100 purchase is $5. how much would the sales tax be on a $40 purchase

$2  The sales tax rate is 5%.    40*0.05= $2.00

To find out how much money gets a sales tax of $1, divide 100 by 5. The answer is 20. For every $20 spent, there is a $1 sales tax. The new purchase is $40. $20 x 2 = $40, so $1 x 2 = $2
There will be a $2 sales tax on a $40 purchase.
I hope this helps!