300 kg of sugar solution has 40% sugar in it. How much sugar should be added to make it 50% in the solution ?

Answer: Option C

In 300 kg of solution,
Percentage of sugar = 40%
∴ Sugar = $$\frac{300\times 40}{100}$$   = 120 kg
Let x kg of sugar be added.
Now, according to the question
$$\begin{array}{rl}& \frac{120+x}{300+x}=\frac{1}{2}\\ & \Rightarrow 240+2x=300+x\\ & \Rightarrow 2x-x=300-240\\ & \Rightarrow x=60\text{kg}\end{array}$$