Multiplying Decimals

3.15 x 1.9 Round to the nearest whole number to estimate answer 3 x 2 = 6 So our answer should be somewhere around 6 Do the multiplication (just ignore the decimals for now) 1 4 315 x 19 28 3 5 3 1 5 5 9 8 5

6 5 9 8 5 . Our estimate Answer before doing anything with decimals Based on our estimate, where do you think the decimal should go? Let’s look at another example to see if we can come up with a rule…..

2.7 x 1.1 Round to the nearest whole number to estimate answer 3 x 1 = 3 So our answer should be somewhere around 3 Do the multiplication (just ignore the decimals for now) 27 x 11 2 7 2 7 2 9 7

Based on our estimate, where do you think the decimal should go? 3 Answer before doing anything with decimals 2 9 7 . Based on our estimate, where do you think the decimal should go? One more example…..

4.125 x 1.8 Round to the nearest whole number to estimate answer 4 x 2 = 8 So our answer should be somewhere around 8 Do the multiplication (just ignore the decimals for now) 1 2 4 4125 x 18 33 4 1 2 5 7 4 2 5

8 7 4 2 5 0 . Our estimate Answer before doing anything with decimals Based on our estimate, where do you think the decimal should go? Ok, lets look at all of our examples…..

Number of Decimal Places in problem Number of Decimal Places in answer Problem Answer 3.15 x 1.9 3 5.985 3 2.7 x 1.1 2 2.97 2 4.125 x 1.8 4 7.4250 4 Look at the number of places after the decimal point in both the problem and the answer……

How to multiply decimals 1 2 3 Estimate the products first. Multiply (ignore decimal) Count the # of digits in decimal places in the factors. That’s the number of decimal places in the product. 123 4.235 1.8 x x 4.235 6 1. x 4.235 6 7 6230 1. 4 4 2 . 76230 8 1234 Copyright © 2013 by Monica Yuskaitis

Solve this problem 3 2 9. 8 7 x . 4 3 9 . 4 8

Solve this problem 5. 2 3 . 2 1 x 5 2 3 + 1 0 4 6 x 1 . 9 8 3

7 6. 8 x 4. 3 5 3 8 4 2 3 4 x 3 0 7 2 x x 3 3 4 . 8 Solve this problem 1 3 8 4 1 2 3 4 x 3 0 7 2 x x + 3 3 4 . 8

Problem Solving

George lived 13. 7 miles from his school. He walked 1/2 ( George lived 13.7 miles from his school. He walked 1/2 (.5) of the way. How far did he walk? 13.7 miles

.5 of 13.7 The first thing to remember is “of” means multiply in mathematics. of x =

.5 x 13.7 1 3 1 3.7 . 5 x 6 . 8 5 miles