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 %