Task 361

The task

In the diagram, the area of rectangle $ABCD$ is $40$. What is the area of $MBCN$? [asy] import olympiad; pair a = (0, 0); pair m = (4, 0); pair b = (8, 0); pair c = (8, -5); pair n = (6, -5); pair d = (0, -5); draw(m--n); draw(a--b--c--d--cycle); label("$A$", a, NW); label("$B$", b, NE); label("$C$", c, SE); label("$D$", d, SW); label("$M$", m, N); label("$N$", n, S); label("$4$", midpoint(a--m), N); label("$4$", midpoint(m--b), N); label("$2$", midpoint(n--c), S); [/asy]