if you let p= # of pencil boxes  and  e= # of eraser boxes, then p + e = 24

if you a p # of pencil boxes at $10 per box and an e # of eraser boxes at $4 per box, then you will pay a total of $210:

                            ($10)*(p) + ($4)*(e) = $210 ===> 10p + 4e = 210

system of equations to solve for p and e:      

                                                              10p + 4e = 210

                                                                 p +  e  = 24