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Hi,
In a classroom there are two kinds of seats; single and double. 
There are twice as single seats as double seats .
.
THe room can accomodate 76 pupils. How many each kind of seats are there?
Question states*** 
Students = Students
      76 = single seats + 2(double seats)
      76 = 2x + 2(x)
      76 = 4x
       x = 19 double seats and 38 single seats
CHECKING our Answer***
 38 + 2*19 = room for 76 Students