Question

The cost of petrol rises by 2 cents a liter. last week a man bought 20 liters at the old price. This week he bought 10 liters at the new price. Altogether, the petrol costs $9.20. What was the old price for 1 liter?​

Answer 1

Let price before rising be x

Petrol bought before rise = 20L

cost of the petrol bought before rise = 20x

now,

price after rising = x+0.02

Petrol bought before rise = 10L

cost of the petrol bought before rise = 10(x+0.02) = 10x + 0.2

We know that,

total petrol bought is worth = 20x + (10x+0.2) = 30x + 0.2 = $9.20

⇒ 30x = 9.18

⇒ x = 9.18/30

⇒ x = $0.306

∴ The cost of petrol/litre before rise is $0.306 or (100 cents and 6 dimes).

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Answer 2

Answer:

We have,

20 liters bought with the old price (a) and 10 liters with the new price (a+2). This is translated into this equation where a is the old price.

$20 \times a + 10(a + 2) = 920(cent). \\ 20 \times a + 10a + 20 = 920(cent). \\ 20a + 10a + 20 = 920(cent). \\ 30a + 20 = 920(cent). \\ 30a = 920 - 20(cent). \\ 30a = 900(cent). \\ a = \frac{900}{30} . \\ a = 30 \: percent.$