You would multiply the students by the price (12*10=120) then multiply chaperones by price (6*15=90) then add both prices together to get 210. Then you would subtract 210 from 380 to get how many boys are in the class. 380-210=170. Since each student cost 10 so you would divide 170/10=17. There would be 17 boys in your class.
A system has no solution if the 2 (or more) functions that make up the system do not intersect. The only pair of functions here that do not have an intersection is found in D. You could graph these 2 on a graphing calculator to see that this is true.
