Using LINEST function with conditions

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[h=2]Using LINEST function with conditions[/h]

How would I apply conditions to data that crosses thresholds. I want to include only the cells that have satisfied condition of n>30 threshold in the LINEST formula. So after each "data sample refresh" I may have to expand or decrease the range used in the LINEST function.

Ideally, I would have one array selected for the LINEST and I would not have to manually increase/decrease the cells used for the function as my column thresholds are crossed.

n = No of vehicles (if n>=30 then the repair $ amount is displayed)
Top row = mileage band
Repair = costs to repair in that mileage band

LINEST Example: Repair

=LINEST(C7:H7,$C$2:$H$2^{1;2},,TRUE)

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	50000	100000	150000	200000	250000	300000	350000	400000
	50k	50-100k	100-150k	150-200k	200-250k	250-300k	300-350k	350-400k
Repair	$1,198.54	$1,054.26	$1,352.07	$1,767.28	$1,271.92	$1,722.23	TOO FEW	TOO FEW

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