The dollar value of the 20% coupon will increase if the discount is from a larger amount. Applying the $25 coupon first reduces the amount that the 20% applies to, so reduces the dollar value of the 20% coupon.
Michelle used the 20% coupon first, so its value was 20%($100) = $20.
Natasha used the $25 coupon first, so the value of the 20% coupon was ...
