help

Here is how I solve it:

Say all the tix were bought in advance: that would mean 1200 dollars were sold. For each ticket we turn into bought at the door, we gain 5 dollars. The difference of 1390 and 1200 is 190. So 190/5 38 were bought at the door. So 82 were bought in advance.

Check

38*15=570

82*10=820

820+570=1390

We can solve this using a system of equations. 

Let $x=$tickets bought at the door

Let $y=$tickets bought in advance

Our equations are 

$10y+15x=1390$

$y+x=120$

We can solve elimination (Feel free to use a different strategy):

Multiply the bottom equation by $10$

$10y+10x=1200$

Subtract the bottom equation

$5x=190$

Divide both sides by $5$

$x=38$

Plug that value into the bottom equation (or the top if you want)

$y+38=120$

Subtract $38$ from both sides

$y=82$

$82$ tickets were bought at the door.

Hope this helped!!!

-Ako