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.
