Melissa wrote this expression to describe the number of pencils she used during the school year.  which situation could be described by this expression?   a. melissa knows she used 15 fewer pencils this year. she does not know how many she used last year.   b. melissa knows she has 15 pencils left. she doesn't know how many she started with.   c. melissa knows that she started with 15 pencils. she does not know how many pencils she used.   d. melissa knows she has 15 pencils left. she used half of her pencils.