Holton Company makes three products in a single facility. Data concerning these products follow:

Product

A B C

Selling price per unit $ $151\mathrm{.}80$ $ $75\mathrm{.}40$ $ $179\mathrm{.}00$

Direct materials $ $66\mathrm{.}00$ $ $42\mathrm{.}00$ $ $107\mathrm{.}60$

Direct labor $ $47\mathrm{.}50$ $ $13\mathrm{.}40$ $ $31\mathrm{.}40$

Variable manufacturing overhead $ $9\mathrm{.}10$ $ $4\mathrm{.}40$ $ $14\mathrm{.}60$

Variable selling cost per unit $ $23\mathrm{.}90$ $ $3\mathrm{.}20$ $ $9\mathrm{.}00$

Mixing minutes per unit $42\mathrm{.}402\mathrm{.}002\mathrm{.}00$

Monthly demand in units $3,0001,0002,000$

The mixing machines are potentially the constraint in the production facility. A total of $14,000$ minutes are available per month on these machines.

Direct labor is a variable cost in this company.

The company should be willing to pay up to the contribution margin per hour for the marginal job, which is $$7\mathrm{.}80.$ How to get $$7\mathrm{.}8?$ Please give me the solution.