high-low

A company,which manufactures a range ofproducts,has decided to introduce a product costing system.As a first step it wishes to analyse the behaviour of its costs.
The following data is available for the previous four periods:
A B C D E

1. Period1 Period2 Period3 period4
2.Total costs $214559 $239970 $243183 $259541
3.Total output 64200 79350 86170 85620

Using high -low method,which one of the following formulae will correctly calculate the cost of the variable element per unit?The answer was =85620-64200/259541-214559…but we know in high-low method we take high quantity and low quantity.But here they didn’t.Why?