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Multi-step problems with addition and subtraction

Multi-step problems with addition and subtraction

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Summary

Multi-step problems with addition and subtraction

​​In a nutshell

Addition and subtraction problems can involve multiple calculations. Knowing the order in which to do the calculations is essential.



Addition and subtraction methods

With multi-step problems, you can apply any addition and subtraction methods you already know. You will have come across two methods: using mental arithmetic, perhaps involving number lines, and using the column method.



Order of calculation

When just adding, you can add the numbers in any order. But, when subtraction is involved, you should read the problem from left to right and calculate in that order.


Alternatively, you can recognise that subtracting a number is the same as adding the negative of that number.


Example 1

Since subtraction can be expressed as addition, the following expressions mean the same thing:


525-2​​


5+(2)5+(-2)


2+5-2+5​​


The point here is that when you subtract the number xx, it is the same as adding the number x-x. If you convert your subtractions into the addition of negative numbers, then the order doesn't matter.


Example 2

Rebecca has written one book, Sally has written three and Joe has also written three. Philip has written 4444 books. How many books have these four authors written combined?


Since you are adding the four numbers, the order doesn't matter. Start with 1+31+3. This can be done mentally with a number line:


+1+1+11234\begin{array}{ccccccccccccc}&+1&&+1&&+1& \\& \curvearrowright &&\curvearrowright&&\curvearrowright \\\underline1&&2&&3&&\underline4 \\|&&|&&|&&| \\ \hline\end{array}​​


So far you are up to 44. Next add the other 33:


+1+1+14567\begin{array}{ccccccccccccc}&+1&&+1&&+1& \\& \curvearrowright &&\curvearrowright&&\curvearrowright \\\underline4&&5&&6&&\underline7 \\|&&|&&|&&| \\ \hline\end{array}​​


Now you are up to 77 books. Finally add Philip's 4444 books. This can be done with the column method:


017+4451\begin {array} {c c c}& \overset{1}{0}& 7 \\+ & 4 & 4 \\ \hline& 5 & 1 \\ \hline\end {array}​​


Hence between these four authors, 51\underline{51} books have been written.


Example 3

Katie is collecting stamps. She currently has 3131. She makes a deal with her friend, that she swaps 44 of her stamps for 88 of her friend's stamps. She then buys 1414 more stamps, before binning 33 because they are repeats. How many stamps does she have now in her collection?


The first step is to 'translate' the worded question into a mathematical statement. So, you want to calculate:


314+8+14331-4+8+14-3​​​


The 3131 is the initial number of stamps she has. The 4-4 accounts for the stamps she gives away in the swap and the +8+8 accounts for the stamps she gains in the swap. The +14+14 comes from the stamps she buys and the 3-3 comes from the stamps she gets rid of at the end.


Since there is addition and subtraction, you should calculate from left to right. Starting with the 31431-4​:


11112728293031\begin{array}{cccccccccccccc}&-1&&-1&&-1&&-1 \\& \curvearrowleft &&\curvearrowleft &&\curvearrowleft&&\curvearrowleft \\\underline{27}&&28&&29&&30&&\underline{31} \\|&&|&&|&&|&&| \\ \hline\end{array}​​


Next add the 88:


217+835\begin {array} {c c c}& \overset{1}{2}& 7 \\+ & & 8 \\ \hline& 3 & 5 \\ \hline\end {array}​​


Now add the 1414:


35+1449\begin {array} {c c c}& {3}& 5 \\+ & 1 & 4 \\ \hline& 4 & 9 \\ \hline\end {array}​​


Finally, subtract the 33:


11146474849\begin{array}{cccccccccccccc}&-1&&-1&&-1&&& \\& \curvearrowleft &&\curvearrowleft &&\curvearrowleft&&\\\underline{46}&&47&&48&&\underline{49} \\|&&|&&|&&|\\\hline\end{array}​​


So Katie has 46\underline{46} stamps in her collection now.


Note: In this problem, both mental maths and the column method were used. This is ok - whichever method you decide is more suitable and preferable for the part of the problem is the best one.



A shortcut

With some multi-step problems, the numbers involved can be manipulated to make the calculations easier.


Example 4

Calculate 

39+54+2139+54+21​​


Here you are adding all the numbers, so the order doesn't matter. But also notice that the 11 from the 2121 would make the 3939 a whole number (4040):


39+54+21 =39+54+20+1 =39+1+54+20 =40+20+54 =60+5439+54+21\\\space\\=39+54+20+1\\ \space\\=39+1+54+20\\\space\\=40+20+54\\\space\\=60+54​​


So with a little rearranging, the multi-step problem has become a single addition.

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FAQs - Frequently Asked Questions

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