Use the formula y=mx+b.

Question

Suppose that the water level of a river is 34 feet and that it is receding at a rate of .5 foot per day. Write an equation forthe water level,L, after D days. In how many days will the water level be 26 feet? Use the formula y=mx+b.

Answer 1

We're simply replacing the names of our variables: y = L (water level) and x = D (days)

We're given the slope value of -0.5 and the y-intercept of 34 (since that's our starting point). This means our equation looks like:

L = -.5D + 34

 

Now to solve this for number of days, we're given L = 26 (the water level is 26 feet), so we put that in our equation and then solve for D:

26 = -.5D + 34

subtract 34 to both sides:

26 - 34 = -.5D + 34 - 34

-8 = -.5D

Divide both sides by -.5 (or multiply by -2)

-8 / -.5 = -5.D / -.5

16 = D

 

So it will take 16 days to reach 26 feet.