## At what rate is this function increasing?

When x = 16,the rate at which $$\sqrt x$$ is increasing is $$\frac {1}{k}$$ times the rate at which x is increasing. What is the value of k?
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 I thought it would be 4 but the answer is 8.
 Its the ratio of the derivatives evaluated at x=16, (2 sqrt(x))^-1