Question 5388
Let x = original cost
Let .90x = the amount the student pays (100%-10% = 90%)
Let .85(.90x) = the sales price (100%-15%=85%)

If the sales price = 306, then

.85(.90x)=306
.765x=306
x=$400

So the original cost was $400
This makes sense because $40 was taken off for the student discount, bringing the sales price to $360 and an additional 15% of 360 = $54, so $360-$54 = $306