This is a problem of consecutive discounts. First discount is 15% and second discount is 20%.
Tha way of solving this problem is by multipliying by the two factors.
When you discount 15% the factor is 100% - 15% = 85% = 0.85
When you discount 20% the factor is 100% - 20% = 80% = 0.80
The the combined factor is: 0.85*0.80 = 0.68
Now apply that discount over the original price to obtain the final price: $23.99 * 0.68 = 16.31
Which means that Lyell saved $ 25.99 - $16.31 = $7.68 (option b).
