At the country concert, the ratio of boy to girls = 2 : 7
 Now, there are 250 more girls than boys, how many boys are there in the concert.
 the equation would be like this:
 Let x = girls
 Let y = boys
 => y /x = 2/7
 => y + 250 = x
 => y + 250 = 7/2y
 => y = 100
 The number of boys is 100
 => x = 100 + 250
 => x = 350, the number of girls.