# The average weight of female infants is at birth 7 pounds. With a monthly weight gain of 1.5 pounds. After how many months does a baby girl weigh 16 pounds?

May 22, 2017

See the solution process below:

#### Explanation:

The formula to solve this problem is:

$c = b + g m$

Where:

$c$ is the current weight - 16 pounds for this problem.

$b$ is the weight at birth or start of a period of time - 7 pounds for this problem.

$g$ is the weight change per month - 1.5 pounds for this problem.

$m$ is the age or number in months - what we are solving for in this problem.

Substituting and solving for $m$ gives:

$16 = 7 + 1.5 m$

$- \textcolor{red}{7} + 16 = - \textcolor{red}{7} + 7 + 1.5 m$

$9 = 0 + 1.5 m$

$9 = 1.5 m$

$\frac{9}{\textcolor{red}{1.5}} = \frac{1.5 m}{\textcolor{red}{1.5}}$

$6 = \frac{\textcolor{red}{\cancel{\textcolor{b l a c k}{1.5}}} m}{\cancel{\textcolor{red}{1.5}}}$

$6 = m$

$m = 6$

It will take an average female infant an average of $\textcolor{red}{6}$ months to go from an average birth weight of 7 pounds to 16 pounds if she gains an average of 1.5 pounds per month.