I think it's easiest to find these type of answers by finding the common factors of a number. First 77, they are 1×77 & 7×11.so you can have 11 rocks on 7 shelves or 7 rocks on 11 shelves. The question states the number or rocks on the shelf is more than the number of shelves, so you must have 11 rocks on 7 shelves. You know you have 77 rocks, so the equation must equal 77. I plugged 7 in for s in the equations to determine the correct answer. 7(7+4)=77.
