Let me explain like this .Suppose There are 40 people gathered somewhere for attending an event.Each Person has to be given 2 sweets for their arrival or for showing their presence in the event. Now 40 other woman joined them..Instead of getting two sweets each of them got one sweet.
Solution: 2 sweets × 40 people = 1 sweet × 80 people = 80
2nd Way:
There are 40 rooms in a building.In Each room we have to fix two bulbs. So How many bulbs are there in the building.
Solution →→→2 bulbs × 40 room = 80 bulbs
Diagram is depicted below for two parts.