Rami sold concessions during a concert. he sold bottled water for $3.50 each and bags of trail mix for $4 each. by the end of the evening, rami sold 400 items for a total of $1,475. how many bottles of water and how many bags of trail mix did rami sell?

Price of bottled water - $3.50 each 
price of bag of trail mix - $4 each 
number of bottles of water sold - x
number of bags of trail mix sold - y
cost spent for bottles - 3.5x
cost spent for bags - 4y
total cost;
3.5x + 4y = 1475 > 1)
total number of items
x + y = 400---2)
multiply 2nd equation by 4
4x + 4y = 1600 ---> 3)
Subtract third equation from the 1st 
0.5x = 125
x = 250
when x = 250
x + y = 400
250 + y = 400
y = 150
number of bottles - 250
number of bags - 150

That she received $100