This is a difficult Starter so there is a prize if you can complete it. The prize fund currently stands at 14 pounds.
For each pair of numbers subtract the sum from the product then divide the result by 20 without using a calculator.
3, 2 
7, 6 
14, 13 
15, 14 
10, 16 
16, 13 
11.5, 19 
19.5, 19.5 
At least one of your answers is wrong.
Congratulations
14 pounds = 1 stone.
You have won a stone!
There are plenty of stones around, take your pick.
A Starter which is similar to this one but slightly easier is Multiply Sum by Difference then Divide.
Topics: Starter  Mental Methods
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Previous Day  This starter is for 27 June  Next Day
0.05  1.45  7.75  9.05 
6.7  8.95  9.4  17.0625 
Note to teacher: Doing this activity once with a class helps students develop strategies. It is only when they do this activity a second time that they will have the opportunity to practise those strategies. That is when the learning is consolidated. Click the button above to regenerate another version of this starter from random numbers.
Dividing by twenty is a lot to ask,
But an easier method will help you.
Dividing by ten is a much easier task,
Then divide your answer by two!
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Teacher, do your students have access to computers such as tablets, iPads or Laptops? This page was really designed for projection on a whiteboard but if you really want the students to have access to it here is a concise URL for a version of this page without the comments: Transum.org/go/?Start=June27 However it would be better to assign one of the student interactive activities below. 

Here is the URL which will take them to a related student activity.
See the National Curriculum page for links to related online activities and resources.