Select a linear cost function equation for each of the following conditions. Use y for estimated costs and X for activity of the cost driver.

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Part $1$

a$.$ Direct materials cost is $$1\mathrm{.}40$ per pound.

y $=$ $$1\mathrm{.}40$X

b$.$ Total cost is fixed at $$7,000$ per month regardless of the number of units produced.

y $=$ X $+$ $$7,000$

c$.$ Auto rental has a fixed fee of $$70\mathrm{.}00$ per day plus $$0\mathrm{.}50$ per mile driven.

y $=$ $$70\mathrm{.}00+$ $$0\mathrm{.}50$X

d$.$ Machine operating costs include $$1,600$ of maintenance per month, and $$17\mathrm{.}00$ of coolant usage costs for each day the machinery is in operation.