Perfect competition 
Competitive firm and its total revenue curve

Total revenue (TR) is equal to price (P) times the quantity (Q):

TR = PQ 

Under perfect competition the firm is a price taker.  The price is therefore given and the firm's total revenue (TR) increases by a constant amount for each additional unit sold.  For instance, the total revenue of 2 units is equal to 2 x R20 = R40, the total revenue for 3 units is 3 x R20 = R60, the total revenue for 4 units is  4 x R20 = R80 and so on.

Price per unit
Total revenue
(TR = PQ)
1 20
2 20
3 20
4 20
5 20