Answer:
x + y = 900
5x + 9y = 6500
400 children
Step-by-step explanation:
Let:
Children = x ; adult = y
x + y = 900 - - - (1)
5x + 9y = 6500 - - - (2)
From (1)
y = 900 - x
Put y = 900 - x in (2)
5x + 9(900 - x) = 6500
5x + 8100 - 9x = 6500
-4x = 6500 - 8100
-4x = - 1600
x = 1600 / 4
x = 400