As given, marginal cost is the instantaneous rate of change of total cost at any level of output.
Let M.C be marginal cost, so
M.C=dCdx
It is given C(x)=0.005x3−0.02x2+30x+5000
M.C=ddx(0.005x3−0.02x2+30x+5000)
⇒M.C=0.005(3x2)−0.02(2x)+30
When 3 units are produced, putting x=3,
we get
⇒M.C=0.005(27)−0.02(6)+30
⇒M.C=0.015+30=30.015
Hence, the required marginal cost is Rs.30.02 (nearly).