Tickets to a play cost $5 for adults and $2 for children. if 1,750 tickets were sold for a total of $7,100, how many children’s tickets were sold? can someone explain me how to make this one step by step?

A - quantity tickets  for  adults.  It costs 5a dollars
c - quantity tickets for children. It costs 2c dollars.

There were 1750 tickets sold, so  a+c=1750$.  That's mean  a = 1750 - c. 

Total: 7 100$,  That's mean  5a + 2c = 7 100.  Substitute now  1750-c instead "a" :

5(1750-c) + 2c = 7100
8750 - 5c + 2c = 7100
-3c = -1650    |divide (-3)
c = 550

So a =1750 -c = 1750 - 550 = 1200

There were sold 1200 tickets for adults and 550 tickets for children

2015 they have a perecent