attn mechanical engineers or physic gurus, page-9

Its been a while since high school physics
However, my thoughts are:

the forces have to be equal for the velocity to be constant (i.e forces cancel)

The effective force of the incline is "scaled" by the sine of the angle so

sin(20) x (120 + x) = 100

120 + x = 100 / (sin(20)

x = 172.4 kg

This does not take into account the friction
but as Starbust mentions you need a coefficient not just the fact it is proportional to the weight

Hope this helps