Decrease rate of the shadow 
If a man of height $6ft$ moves with $5ft/sec$ velocity towards a lamp hanging at $15ft$ height, then at what rate his shadow will decrease?

My Work: Let the initial distance between the man and the lamp post be $x$ and the length of shadow be $y$.

Then, $\frac{x}{9} = \frac{y}{6} \implies y = \frac{2x}{3}$
Now how do I handle the $5ft/sec$ velocity and the decrease rate of the shadow?
I think the rest part is so easy. But I am missing somewhere.
Note: This is a problem from BdPhO 2016 but completely math related. So I posted it here. 

Anatoly has given a simple solution,
but in case you are expected to use calculus, here's another simple solution.
Starting from $y = \dfrac23 x,\;\; \dfrac{dy}{dt} = \dfrac23 \dfrac{dx}{dt} = \dfrac 23\times -5 = -\dfrac{10}{3}$ ft/s,
the negative sign showing decreasing length of shadow.
