Monica paid sales tax of $2.80 when she bought a new bike helmet. If the sales tax rate was 7%, how much did the store charge for the bike helmet before tax?

7 % of cost price is 2.80, What about 1%?

7 = 2.80

1 = x (where x is the price value of 1% in dollars)

We simplify the above equation, so:

7 * x = 2.8 * 1 → 7x = 2.8 → x = 2.8 / 7 → x = 0.4 dollars

We know that 100% represents the whole price,

so if 1% is 0.4 dollars, then 100% will be 0.4 * 100 = 40 dollars

Therefore the store charged $40 dollars for the helmet before tax