Adding and Subtracting Decimals

We usually add numbers when we have different and varied data, but when we are given with a set of repeated data, we multiply.

Multiplication is a much easier way to calculate repeated data.

2 × 3 also means 2 + 2 + 2 which is equal to 6.

Example: Let’s say Jim buys 4 pens costing $3.50 each. What is the total cost?

                                                                                       3.50

                                                                                       3.50

Cost of one pen = $3.50                                             3.50  

Hence, the cost of 4 pens = $3.50 × 4              +     3.50

                                                                                 _________

                                                                                      14.00

Why is place value important when adding and subtracting decimals? 

Let’s look at this example now:    2.87 + 3.02 + 0.006 + 123.9 + 4

Example: Renée bought 6.75 pounds apples, 4.5 pounds oranges, 3.82 pounds bananas and 2.25 pounds grapes. How much of fruits did she buy in all?

Solution: Fruits Renée bought in all = 6.75 + 4.5 + 3.82 + 2.25 = 17.32 pounds

How do you subtract with decimals?

Subtracting decimals is much similar to the way we have learnt above in adding decimals. Let’s see how.

Example: Daniel has 2 pieces of wood measuring, 10.34 inches and 7.56 inches. How much longer is the first piece than the second one?

Subtract the lengths of 2 given wooden pieces.                                    10.34

                                                                                                                       –7.56

                                                                                                                     _________

                                                                                                                        2.78

So, the first piece is 2.78 inches longer than the second wooden piece.

Check point

Add the following decimals:

1. 3.25 + 1.32 + 4.67
2. 1.46 + 2.35 + 1.00

Subtract the following decimals:

6. 5.82 – 1.23
7. Richard has 100.56 pounds of grain out of which he 34.156 pounds to his brother, John. How many pounds of grain is Richard left with?
8. A box of peanut butter cookies cost $3.40 and a box of oatmeal cookies cost $4.25. What is the total cost for both cookies boxes?
9. Mary has 20.752 inches of ribbon out of which she used 12.456 inches of ribbon for decorating purpose. How much ribbon is she left with?                                                        

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