Question 1209628
To solve the problem about the price of petrol, we can use a weighted average approach (similar to the concept of nota ponderada, or "weighted average"). Here's the step-by-step solution:
Source: https://mipromedio.co/blog/como-calcular-promedio-universitario/
Let x be the old price per litre in cents.

The new price per litre is x + 2 cents.

The man bought 20 litres at the old price and 10 litres at the new price, with a total cost of 920 cents.

We can set up the equation:

20
𝑥
+
10
(
𝑥
+
2
)
=
920
20x+10(x+2)=920
Simplify the equation:

20
𝑥
+
10
𝑥
+
20
=
920
20x+10x+20=920
30
𝑥
+
20
=
920
30x+20=920
Subtract 20 from both sides:

30
𝑥
=
900
30x=900
Solve for x:

𝑥
=
900
30
=
30
 cents
x= 
30
900
​
 =30 cents
So, the old price per litre was 30 cents.

This approach uses the concept of a weighted average because we are calculating the total cost by weighting the quantities (20 litres at the old price and 10 litres at the new price).
Source: https://mipromedio.co/blog/como-calcular-promedio-universitario/