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In an ice-cream shop there was some money in a drawer. After selling $6$ ice-cream cones, there are $70$ dollars in the drawer. After selling a total of $16$ ice-cream cones, there are $120$ dollars in the drawer. How many dollars were there in the drawer at the start? (2021 Math Kangaroo Problem, Level 3-4, Question \#13) 
Overseas Competition->Knowledge Point->Calculation Modules
The price of one ice-cream cone is $(120-70)\div(16-6)=5$ dollars. The price of $6$ ice cream cones is $5\times6=30$ dollars. There were $70-30=40$ dollars at the start. 