Answer :
x = # of boys, y = # of girls
x + y = 10 --> y = 10-x
2.25y + 1.75x = 20.50
2.25(10-x) + 1.75x = 20.5
22.5 - 2.25x + 1.75x = 20.5
-0.5x = 20.5-22.5
-0.5x = -2.0
÷ -.5 ÷ -.5
x = 4 --> 4 boys, so 10-4 = 6 girls
x + y = 10 --> y = 10-x
2.25y + 1.75x = 20.50
2.25(10-x) + 1.75x = 20.5
22.5 - 2.25x + 1.75x = 20.5
-0.5x = 20.5-22.5
-0.5x = -2.0
÷ -.5 ÷ -.5
x = 4 --> 4 boys, so 10-4 = 6 girls