Tickets for a concert cost $2 each for children and $5 each for adults. a group of 30 people consisting of children and adults paid a total of $87 for the concert. how many adults were in the group?

Let,
the no. of adults be "x"
then, the no. of childern = total people - total adults 
                                   = (30 - x)
Now,
According to the quesiton,
2 (30 - x) + 5x = 87
60 - 2x + 5x = 87
60 + 3x = 87
3x = 87 - 60
3x = 27
x = 27 / 3
x = 9

So, there were 9 adults in the group