The reduced levels of points, 30 metres apart along the longitudinal section of a road portion between chainages 5 and 9 are shown in the given figure. If there is a uniform up-gradient of the road 120 in 1, the chainage of the point with no filling or cutting is

Gradient given is 1:120
So for 30m rise of = (1/120*30),
= 0.25 m.

RL of formation level @6 chain = 100.0 + 0.25.
= 100.25 m.
RL of formation level @7 chain = 100.25+ 0.25
=100.5 m.

So difference = 102-100.5 = 1.5.

By similar triangles -
(1/x) = 1.5/(30-x)
X = 12.

So the answer is (6+12) chains.

How? Please solve.