A single discount equivalent to three successive discounts of 15%, 20% and 40% on the marked price of an article is:

The single equivalent discount is 59.2%.

Each discount leaves a factor: 0.85, 0.80 and 0.60.

Multiply them: 0.85 x 0.80 x 0.60 = 0.408.

So the customer pays 40.8% of the marked price.

Effective discount = 100% - 40.8% = 59.2%.

Successive discounts multiply as remaining fractions.