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2016-01-24T02:33:42+08:00

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Total adult and children = 12 people

   x + y = 12
   y = 12 - x

Number of children   ⇒       x
Number of adults      ⇒      12 - x 

Admission Fee paid by each group:
     Children:  30 (x)      ⇒    30x
     Adults: 70 (12-x)     ⇒    840 - 70x
     Total Admission fee paid = 640

Equation:  Solve x (number of children)
    30x + 840 - 70x = 640
    -70x + 30x = 640 - 840
    -40x = -200
    -40x/-40 = -200/-40
    x = 5

ANSWER:  The number of children is 5.

Substitute 5 for for number of adults:
   Adults: 12 - x = 12 - 5  ⇒  7 adults
                
Check:
   5 + 7 = 12
   12 = 12   (True)

Total admission fee:
    30 (5) + 70 (7) = 640
    150 + 490 = 640
    640 = 640    (True) 
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