I don’t know how to explain on why the student was incorrect

the constant proportionality means the ratio between two directly proportional quantities.

so the constant proportionality will be 7.5÷6=1.25 or 10÷8=1.25 or 12.5÷10=1.25

constant proportionality should be same all through because it's a constant

$15 each

step-by-step explanation:

$45/3 children= $15 each

check above link.

graphically?   well, you graph them both, the solutions is where they both intersect each other, since there are two solutions, meaning they will meet twice, or namely the linear will run into the quadratic curve twice, and where they meet, those x,y coordinates are the solutions to the system.