math
posted by Helen .
An instrument store gives a 10% discount to all students off the original cost of an instrument. During a back to school sale an additional 15% is taken off the discounted price. Julie, a student at the local high school, purchases a flute for $306. How much did it originally cost?
i know its absurdly simple but my brain feels like mush right now

Do it step wise as the reverse of the original purchase.
So to get back to the discounted price (after the 10%) then multiply the £306 by 1/0.85. Then to get to the original price multiply this result by 1/0.9. 
100%15%= 85% 306/85%= 360
100%10%= 90% 360/90%= 400
Hope this helps