Alice is planning to open up a sandwich shop. An estimate of her costs/revenues are as follows: 

average sales price per sandwich: $12.75;

 yearly rent: $24,000;

 monthly fixed utility bill: $800; 

average cost of ingredients per sandwich: $6.55;

 monthly labour bill (fixed): $10,500; 

miscellaneous fixed supplies/month: $100; 

misc. variable supplies: $1.45 per sandwich. 

How many sandwiches does she need to sell per month to make an operating income of $139,940 per year?