Given:
A bookstore can purchase several calculators for a total cost of $720
Let the number of calculators = x
And the cost of one calculator = y
So,

If each calculator costs $1 less, the bookstore could purchase 10 additional calculators at the same total cost.
so,

divide the equation (2) by equation (1)

Substitute with x into equation (1) then solve for y:

Substitute into equation (1) to find x:

So, the answer will be:
The number of calculators can be purchased at the regular price = 80