In Boeing 757, there are 50 tourist, 30 economy, and 32 first class seats. .
In Boeing 767, there are 75 tourist, 45 economy, and 50 first class seats.
In Boeing 777, there are 40 tourist, 25 economy, and 30 first class seats.
50x+75y+40z=305
30x+45y+25z=185
32x+50y+30z=206
after solving this you can gey x,y and z
x for 757
y for 767
and z for 777
