If 11 oranges cost $2.35, we could calculate the cost of each of these oranges, simply dividing the total cost ($2.35) between the amount of oranges bougth (11). This will give us the cost of a single orange, that is, the unitary cost= .
Then, if we want to know the cost of 18 oranges, we just have to multiply the cost of a single orange (0.21) by 18: 0.21x18 = 2.85