Jarett's puppy weighed 3 3/4 ounces at birth at one week old, the puppy weighed 5 1/8 ounces at two weeks old the puppy weigh at three weeks old?

Well, i find it helps to turn the fractions into decimals. 
3 3/4 = 3.75
5 1/8 = 5.125 

so now subtract: 
5.125 - 3.75 = 1.375 

so that means it weighs 1.375 ounces more each week

so then take the second week (5.125) and add on 1.375

5.125 + 1.375 = 6.5 

now to convert that into a fraction (lowest terms) it would be: 
6 1/5 ounces 

Therefore, 
Jarett's puppy will weigh 6 1/5 ounces at three weeks old. 

Hope this helps and ave a great day! :D

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