Linear Equation

Question

A baby has its first doctor's visit when it is 4 months old and it weighs 16 pounds. The doctor tells the mother to expect the baby to gain 2 pounds each month.

Find an equation in the form  that models the baby's weight, where  is the age of the baby in months and  is its weight in pounds.

Answer: y =

Answer 1

The stumbling block in this problem is the fact that the baby is already 4 months old at first doctor's visit.

You must take this into consideration when writing the equation.

In y=mx+b form (slope-intercept), b is the initial value of y at the first doctor's visit and m is the rate of change (2 lbs per month, in this case).

You may think that y = 2x + 16 is the correct set-up, but that would only work if the baby weighed 16 pounds at birth, or x=0 months.

To account for the baby already being 4 months old, we need to subtract 4 from the x value in the equation:

y = 2(x-4) + 16

Then simplify the equation

y = 2x - 8 + 16

y = 2x + 8

Hope this helps!