Index number practice question from acca study hub

The table below contains details of an airline’s expenditure on aviation fuel:

Year Total expenditure on aviation fuel Total distance flown Fuel price index
$m km million
1 600 4,200 120
2 1,440 4,620 240
The following statements relate to the changes between Year 1 and Year 2.

The quantity of fuel consumed increased by 140%
The quantity of fuel consumed increased by 20%
The quantity of fuel consumed per km flown increased by 20%
The quantity of fuel consumed per km flown increased by 109%
2. Which statements are true?

A.1 only
B.2 only
C.2 and 3 only
D.2 and 4 only
The correct answer is B.

((1,440 × 120 ÷ 240 ÷ 600) ? 1) × 100 = 20%

I have read the lecture and read notes for this chapter and have also studied the notes provided on the acca study hub website. I don’t understand how they arrived at the answer and logic that was used. Please could you help me understand?

Question 2:

The following information relates to a company’s semi-variable production overheads:

Year Output Overhead Relevant
Units $ price index
20X2 1,000 12,000 130
20X3 1,200 14,000 140
4. What is the variable overhead cost per unit, expressed in 20X3 prices?

A.$5.00
B.$5.38
C.$10.00
D.$11.67
The correct answer is B.

Using the high low method to calculate variable cost per unit:

Change in cost ÷ change in output = ($14,000 ? $12,923 (W)) ÷ (1,200 units ? 1,000 units) = $5.38

WORKING

20X2 overhead cost expressed in 20X3 prices = $12,000 × 140/130 = $12,923.

I first found out the VC per unit by using high low method, then applied the following formula 10*140/130 and got 10.76 as the variable cost expressed in the 20X3 prices which is what they have asked.