Ok so we going to cross multiply. 
We going to put it in a fraction form. People / boxes. 
20 people 50 people
————— = —————-
6 boxes X 
Cross multiply 
20 x (x) = 50 x 6
20x = 300 Now divide 20 on both sides 
—— ——-
20 20
X = 15 
Final answer : 15 boxes will feed the 50 people.