Think about it... If it gains 4 pounds in 16 years, how many does it grow in 1 year? :) See the relation? If not...
a) How would you express it growing per year? 4 pounds in 16 years... It must be 4/16!
b) To find the unit rate, the numerator MUST BE 1. What can you do to 4 to get to 1? You can divide by 4!
c) Of course, whatever you do to one side of the fraction, you HAVE to do to the other. That means you have to divide the 4 in the equation by 4 AND the 16 in the equation by 4. You should get 1/4, because 4/4 is 1 and 16/4 is 4.
d) Wow! That means your unit rate is 1 pound per 4 years, or 1/4!
TIP:
Try thinking of the division sign / as 'per'. Instead of the unit rate being thought of as 1/4, think of it as 1 per 4 years. :)