Trey Monson starts a merchandising business on December $1$ and enters into the following three inventory purchases. Monson uses a perpetual inventory system. Also, on December $15,$ Monson sells $15$ units for $$20$ each.

Purchases on December $710$ units @ $ $6$ cost

Purchases on December $1420$ units @ $$12$ cost

Purchases on December $2115$ units @ $$14$ cost