Let, x, represent the value of Dave's markers. You then make the equation (5x+x=18) because both markers combined is 18 and Sheila has 5 times the markers of Dave so her value would represent 5x. Solving the equation would get you x=3 BUT THAT IS THE VALUE OF DAVE'S MARKERS! The question asks for Sheila's with is 5*3. So the answer is 15 markers.
