The discount series 10%, 20%, 40% is equivalent to a single discount of = ?

Answer: Option B

Let the price is = Rs. 100
After discount price
$$\begin{array}{rl}& \text{ = 100}\times \frac{90}{100}\times \frac{80}{100}\times \frac{60}{100}\\ & \text{ = Rs}\text{. 43}\text{.2}\\ & \text{Difference = 100}-43.2\\ & =56.8\%\end{array}$$
So, 56.8% is a single discount of percent of this series.

Alternate :
Successive discount of 10% of 20%
$$\begin{array}{rl}& =10+20-\frac{10\times 20}{100}\\ & =28\%\end{array}$$
Then successive discount of 28% and 40%
$$\begin{array}{rl}& =28+40-\frac{28\times 40}{100}\\ & =56.8\%\end{array}$$