Calculation of Variable cost per unit – High Low method

Hello Sir..! Good day to you..

I have doubt relating to the following question from ACCA Study Hub.

The following shows the total overhead costs for given levels of a company’s total output:

Output
units Cost
$
1,000 4,000
2,000 7,000
3,000 10,000
4,000 9,450

A step up in fixed costs of $500 occurs at an output level of 3,500 units.

What would be the variable overhead cost per unit (to the nearest $0.01) using the high low method? (Answer in $.)

The provided answer is as follows:

The correct answer is $1.65.

Variable overhead cost is $1.65 per unit.

Find the change in cost between the highest and lowest output levels not explained by the step in fixed costs ($9,450 ? $4,000 ? $500 = $4,950), and divide by the change in output between the highest and lowest output levels (4,000 units ? 1,000 units = 3,000 units) ($4,950 ÷ (4,000 units ? 1,000 units) = $1.65 per unit.

In the Kaplan Study Text, a different method is adopted when there is a step up in fixed costs which is as below:

Choose the two activity levels where the fixed costs remain unchanged
and calculate the variable cost per unit and the total fixed cost using the
high/low technique.

If we apply this technique in the question provided above, the activity levels where the fixed costs remain unchanged are between 1000 and 3000 units (since step up occurs at 3500 units). So Variable cost per unit would be calculated as:

($10000 – $4000) / (3000-1000)
$3 per unit.

WHICH METHOD IS CORRECT ???

Thanks in advance..!