You multiply 3 and 6 and that equals 18. You multiply the numbers together because you know he has 3 bags of marbles and he has 6 marbles in each bag. Then you take the 18 marbles and add them together with the 37 marbles and you get 55 marbles.
By just looking at ur whole numbers...12 + 3 + 5 = 20...and thats not even counting ur decimals..so by adding ur decimals, u know that it will be over 20.
