Answer: 175
Step-by-step explanation:
Let say
x the number of ticket at $6
y the number of ticket at $10
z the number of ticket at $12
500 total tickets were sold : x+y+z=500
the total income from ticket sales were $4.500: 6*x+10*y+12*z=4500
the combined number of $6 and $10 tickets was four times the
number of $12 tickets sold: x+y=4*z
x=175
Resolution is in the file xls jointed.