Question 581662
Let a = number of adults, and c = number of children



"A group of 60 people attend a ball game" means that a + c = 60.


"There were twice as many children as adults in the group" means that c = 2a


So the system of equations is 

a + c = 60
c = 2a



Now let's solve for c


a + c = 60


a + 2a = 60


3a = 60


a = 60/3


a = 20


So there are 20 adults


c = 2a


c = 2(20)


c = 40


and there are <font color="red">40</font> children



Note: add up the two counts to get 40+20 = 60, so this checks the first condition (that there are 60 people). Also, notice how 40 kids is exactly double 20 adults.