When the price of an article was reduced by 20% its sale increased by

The question asks "what was the net effect on the sale?" and tells us "the sale increased by 80%". So the answer is 80%. There's a problem with the wording of the question.

The question surely means to ask "what was the effect on the revenue". This is a pure ratio question, so you can just invent numbers if you like - the answer will be the same for any numbers you choose. So you could imagine at first 10 items were sold for $10 each, for $100 revenue. Then after the percent changes, 18 items were sold for $8 each, for $144 revenue, a 44% increase.

Or you can use algebra: if n items are sold for $p each, revenue is $np. If we reduce the price by 20%, the new price is 0.8p. If we increase the number sold by 80%, the new number sold is 1.8n. So the new revenue is (0.8p)(1.8n) = 1.44np, which is 1.44 times the old revenue, so is 44% greater.