18. The total weight of a box, 20 plates and 30 cups is $4.8 \mathrm{~kg}$. The total weight of the box, 40 plates and 50 cups is $8.4 \mathrm{~kg}$. What is the total weight of the box, 10 plates and 20 cups?
A $3 \mathrm{~kg}$
B $3.2 \mathrm{~kg}$
C $3.6 \mathrm{~kg}$
D $4 \mathrm{~kg}$
E $4.2 \mathrm{~kg}$
18. A Let the weights in kg of the box, 1 plate and 1 cup be $b, p$ and $c$ respectively.
Then: $b+20 p+30 c=4.8$ (i); $b+40 p+50 c=8.4$ (ii).
Subtracting (i) from (ii): $20 p+20 c=3.6$ (iii). So $10 p+10 c=1.8$ (iv).
Subtracting (iv) from (i): $b+10 p+20 c=3$. So the required weight is $3 \mathrm{~kg}$.