Form two simultaneous equations with the information provided:
10P + 4R = 210, P = no. of boxes of pens, R = number of boxes of rubber bands
10 is the cost of a box of pens; multiply this by P will give you the total cost of all the boxes of pens bought.
4 is the cost of a box of rubber bands; multiply this by R will give you the total cost of all the boxes of rubber bands bought.
We can also say:
P + R = 24, as 24 boxes (cartons) were bought in total.
Solve simultaneously:
P = 24 - R
Sub' this into 10P + 4R = 210 to give:
10(24 - R) + 4R = 210
240 - 10R + 4R = 210
-6R = -30
R = 5
and so:
P = 24 - (5) = 19
19 boxes of pens
5 boxes of rubber bands