SUBTRACTING POSITIVE RATIONAL NUMBERS

We can subtract positive rational numbers using a number line. 

When we subtract a positive rational number from another positive rational number, we have to move in the negative direction on the number line.  

For example, let us consider 4/5 - (+3/2). 

Here we subtract a positive rational number. That is +3/2 or +1.5. So we have to move 1.5 units in the negative direction from 4/5 (or 0.8) on the number line.  

Example 1 : 

Subtract 2.5 from 3.5 using a number line. 

Solution : 

Since we subtract a positive rational number 2.5 from 3.5, we have to move 2.5 units in the negative direction from 3.5 on the number line as given in the picture below.

After having moved 2.5 units in the negative direction, we are in the position of "1"

So,

3.5 - (+2.5)  =  1

Example 2 : 

Subtract 1.5 from 4.5 using a number line. 

Solution : 

Since we subtract a positive rational number 1.5 from 4.5, we have to move 1.5 units in the negative direction from 4.5 on the number line as given in the picture below.

After having moved 1.5 units in the negative direction, we are in the position of "3"

So,

4.5 - (+1.5)  =  3

Example 3 : 

Subtract 5 from 3 using a number line. 

Solution : 

Since we subtract a positive rational number 5 from 2, we have to move 5 units in the negative direction from 2 on the number line as given in the picture below.

After having moved 5 units in the negative direction, we are in the position of "-2"

So,

3 - (+5)  =  -2

Example 4 : 

The temperature on an outdoor thermometer on Monday was 5.5 °C. The temperature on Thursday was 7.25 degrees less than the temperature on Monday. What was the temperature on Thursday ?

Answer : 

Step 1 : 

Find 5.5 - 7.25.

Step 2 :

Start at 5.5.

Step 3 :

Move |7.25| = 7.25 units to the left, because we are subtracting a positive number.

The result is -1.75.

The temperature on Thursday was -1.75 °C.

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