Question

Officials begin to release water from a full man-made lake at a rate that would empty the lake in 10 weeks, but a river that can fill the lake in 20 weeks is replenishing the lake at the same time. How many weeks does it take to empty the lake

Answer 1

Answer:

It takes 20 weeks to empty the lake

Step-by-step explanation:

Some rate, let's call it $R_{empty}$ empties the lake in 10 weeks and a river (with rate $R_{river}$ can fill the lake in 20 weeks. The idea behind the solution of this type of problem is always set the rate to the same units. Let's see this:

$R_{empty}=\frac{1\ lake}{10\ weeks}$ which implies that this rate empties 0.1 of the lake per week, thus, $R_{empty}=0.1/week$.

Using the same idea, we can obtain that the filling rate of the river is:

$R_{river}=\frac{1\ lake}{20\ weeks} = 0.05 /week$

Since both rates are acting at the same time, the total rate of flowing water (either in or out the lake) would be given by:

$R_{total}=R_{empty}+R_{river}\\R_{total}= -0.1 + 0.05=-0.05/week$, which means that the lake is being empty at a rate of -0.05 week and using the rule of three:

$\frac{0.05}{1} = \frac{1}{total\ of\ weeks} \\total\ of\ weeks = \frac{1}{0.05}=\frac{1}{\frac{1}{20} }\\ total\ of\ weeks = 20$

Thus, It takes 20 weeks to empty the lake.