If 7 liters of petrol cost $14.70, calculate the cost of 15 liters of petrol.

The cost of 15 liters of petrol if 7 liters cost $14.70 is $31.50

To calculate the cost of 15 liters of petrol if 7 liters of petrol cost $14.70,

First, we will determine the cost of 1 liter of petrol.

From the question,

7 liters of petrol cost $14.70

That is,

7L = $14.70

Then,

1 L = $14.70 ÷ 7

1 L = $2.10

∴ The cost of 1 liter of petrol is $2.10

Now, for the cost of 15 liters of petrol, we will multiply the unit price ($2.10) by 15

That is,

The cost of 15 liters of petrol = 15 × $2.10

∴ The cost of 15 liters of petrol = $31.50

Hence, the cost of 15 liters of petrol if 7 liters cost $14.70 is $31.50

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Hi

7 = 14.70

15= X

so :  X =  15*14.70 /7 = 31.5

15 liters  of petrol cost  31.5

How I would solve this is algebraically. See below:
(Multiply by 7 to get rid of denominator on right side.
(Multiply by 2 to get rid of denominator on left side.

(subtract 2n from both sides)
(divide both sides by 5)