A store is having a 20%-off sale on all merchandise.

a) If Mai buys one item and saves $13, what was the original price of her purchase? and explain

This is easy. There is a 20 percent discount which means now it's only (100-20%) discount.

This is 80% discount.

Money saved is equal to 100-80%

=20%

And that 20% is equal to 13$ as per the passage.

So now we need to calculate 1 percent of the item without discount.

This is equal to 13/20

This is equal to. 65

Now to find the original price you will need to multiply. 65 to 100 this is equal to 65$.