Frank sold 4 hamburgers and then sold 2 more, so all together Frank already sold 4+2= <<4+2=6>>6 hamburgers
If each hamburger is $5 and he sold 6 hamburgers already, Frank has made 5*6= <<5*6=30>>30 dollars.
Frank wants to make $50 and has already made $30, so Frank still needs 50-30= <<50-30=20>>20 dollars.
Since he needs $20 and each hamburger is $5, Frank needs to sell 20/5= <<20/5=4>>4 more hamburgers
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