High Low method with Price Index

MONTH……………UNITS…………OVERHEAD…………. INDEX
JAN………………….10………………….3352………………….100
FEB………………….10.5………………….3479………………….101
MAR………………….12………………….3860………………….102
APR………………….9………………….3098………………….104
MAY………………….9.5………………….3225 ………………….105
JUN………………….10.25………………….3416………………….106

Q. Using high low analysis, what is the variable cost per unit in June’s prices?
Answer is {(3860 x 106/102) -(3098 x 106/104)} / (12-9) = 284
Please be kind enough to explain how’s this answer is getting in detail.
Thank You