The length of a rectangle is increasing at a rate of 12 in./s, while its width is decreasing at 2 in./s. Find the rate of change ofits area when its length is 25 in. and its width is 5 in.

In this problem, we have that
A=L*W
dL/dt=12 in/sec ----> given
dW/dt=-2 in/sec ---> given
Find out dA/dt
dA/dt=L*dW/dt+W*dL/dt
For L=25 in and W=5 in
substitute
dA/dt=25*(-2)+5*(12)
dA/dt=-50+60