7 is D because one person is doing 4 times as much work as the other person they are working with. The person that is doing the least is labelled m, and is added to 4m because 4m does 4 times as much work as m, except he does it along with m so they MUST be added together. 8 is A because if you rearrange the equation, you get 4s=70. If the 4 shirts would have cost 70$ before applying that coupon, then 4 shirts of the same cost (4s) would equal 70$. Try the last ones yourself so you can make sure you fully understand what is happening.