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.
6, 5 
9, 8 
14, 13 
14, 11 
13, 17 
17, 17 
12.5, 14 
17.5, 17.5 
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
0.95  2.75  7.75  6.45 
9.55  12.75  7.425  13.5625 
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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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