4. One standard balloon can lift a basket with contents weighing not more than $80 \mathrm{~kg}$. Two standard balloons can lift the same basket with contents weighing not more than $180 \mathrm{~kg}$. What is the weight, in $\mathrm{kg}$, of the basket?
A 10
B 20
C 30
D 40
E 50
Solution
B

Two balloons can lift one basket and $180 \mathrm{~kg}$. One balloon can lift one basket and $80 \mathrm{~kg}$. Therefore we can conclude that one balloon can lift a total of $100 \mathrm{~kg}$. Hence the weight, in $\mathrm{kg}$, of the basket is $(100-80)=20$.