Answers

2016-06-19T20:32:20+08:00
56 - 36 = 20

There are 20 pupils who didn't join the field trip.
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2016-06-20T00:07:13+08:00
Let a= number of pupils
      b= number who joined
      c= number of pupils who did not join the field trip 

a= b+c

To get c, 
c= a-b

Therefore,
c=56-36
c= 20

The number of pupils who did not joined the fieldtrip is 20.
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