Let adults = X
Then children would be 2x ( Twice as many as adults, means 2 times the amount of adults)
Now we have adults plus children equal 36:
X + 2X = 36
2X + X = 3x
Now we have 3X = 36
Divide both sides by 3 to solve for X
X = 36/3
X = 12
2X = 2*12 = 24
There were 12 adults and 24 children