There are $3\times20+20\div2=70$ slices of beef. $70\div2=35$ 
Cindy prepares burgers with two slices of beef each. A box of beef has $20$ slices. How many burgers can she prepare with all the three and a half box of beef? (Adapted from 2013 Math Kangaroo Problem, Level 3-4, Question \#7) 
Options:
  * $$10$$ 
  * $$35$$ 
  * $$45$$ 
  * $$25$$ 
  * $$30$$ 