And animals weight change from 12 pounds to 16 pounds what was the percent increase to the nearest whole number

That's a 33% increase.

I calculated this using the formula:



Where n = the new value (16 in your question), o = the old value (12 in your question) and the result is outputted as a percent increase.  You can check that this is correct by finding 33% of 12, adding the result to 12, and checking that the result equals your "new" number.  

Note that 33% is only an approximation as your question requires a number rounded to the nearest whole.

Could you make it more clear?