Look at the following expressions
20 - 30 = ? (subtract 30 from 20)
-24 + 15 = ? (add 15 to negative 24)
-9 - 7 = ? (subtract 7 from negative 9)
There are different ways to solve the problems above but I have seen the number line method works best for most of my students. There is also a fun way to solve them but I will share that only after I explain the number line method.
The idea is, you draw up a number line, start from 0 and go right from current position every time you come across a positive number and you go left each time you see a negative number. Watch the video now.
You can use this method to evaluate any expression having both positive and negative numbers. However, when there is a whole bunch of numbers to work with - both positive and negative - there is always a risk of jumping in the opposite direction or landing up at an incorrect number trying to do it quick. Remember, at each step you need to do a calculation to figure out the new position.
In the next video let me show you another fun way to solve these problems. Some of my students have told me they never use the number line method once they have started using this one.
So, you just had to set up the 'game' between the two teams and that helped you solve Math problems! Did you like it?
Alright, you feel better but have you ever seen expressions like the ones below?
15 +(-21) (add negative 21 to 15)
-42 -(-5) (subtract negative 5 from negative 42)
20 -(+8) (subtract positive 8 from 20)
In the following video I show you how to work them.
In the video above I mentioned about the rule that opposite signs give you a negative and same signs will make the number positive. In another video on another day I will explain why is it so. By now I hope you have a clear idea on how to add and subtract positive and negative numbers.