R = P*X = 6000X-0.4X^3
Profit = Y = 6000X-0.4X^3-24000X-5200
= -0.4X^3-36000X-5200
Given dY / dt = 6384
Find dX / dt when X = 44
d/dt [Y] = d/dt [-0.4X^3-3600X-5200]
dY/dt = -1.2X^2*dX/dt - 3600*dX/dt
= (-1.2X^2-3600)*dX/dt
dX/dt = dY/dt* (-1.2X^2-3600)^-1
when X=44
dX/dt = 6384/(-1.2*44^2-3600) = -1.0778
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