If 2 succesive discounts a% and b%
then net discount = a + b – (ab/100)
If 3 succesive discounts a%, b% and c%
then single discount
= a + b + c – (ab/100 + bc/100 + ca/100 – abc/10000)
In this case,
single discount
= 10 + 20 + 40 – (200/100 + 800/100 + 400/100 – 8000/10000)
= 70 – (14 – 0.8)
= 56.8 %