148 inches.

We can use Pythagoras' theorem if we draw an imaginary line across to create a right-angled triangle.

The hypotenuse is equal to the rope's length R. The bottom of the triangle is 48 inches. The vertical side is R - 8 (the difference between 20 inches and 12 inches).

Pythagoras' theorem tells us that:

a^{2} + b^{2} = c^{2}

Where c is the hypotenuse.

This gives us:

48^{2} + (R - 8)^{2} = R^{2}

^{ }^{ } 2304 + (R - 8)(R - 8) = R^{2}

^{ } 2304 + R^{2} - 16R + 64 = R^{2}

^{ }^{ } 2304 - 16R + 64 = 0

^{ }^{ } 16R = 2368

^{ }^{ } R = 148

As required. QED.