The formula to solve this problem is:

#c = b + gm#

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 + 1.5m#

#-color(red)(7) + 16 = -color(red)(7) + 7 + 1.5m#

#9 = 0 + 1.5m#

#9 = 1.5m#

#9/color(red)(1.5) = (1.5m)/color(red)(1.5)#

#6 = (color(red)(cancel(color(black)(1.5)))m)/cancel(color(red)(1.5))#

#6 = m#

#m = 6#

It will take an average female infant an average of #color(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.