Gribble Companys high and low level of activity last year was 60,000 units of product produced in May and 20,000 units produced in November. Machine maintenance costs were $104,000 in May and $40,000 in November. Using the high-low method, determine an estimate of total maintenance cost for a month in which production is expected to be 45,000 units.

a. $90,000

b. $96,000

c. $78,000

d. $80,000

how did we solve it and get D?