Question 422302
High efficiency furnace---$2250 + $412 x : x is the number of years
Standard furnace $1710 + 466x

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Cost equal

2250+412x= 1710+466x
2250-1710=466x-412x
540=54x
540/54 = x 
x= 10 years when cost will be equal.

...
For x less than 10 years Standard furnace is economical

Plug 20 for both options 
2250+412*20 =10,490 high efficiency
1710+466*20=11,030 standard
You may decide