a/b = c/d ⇔ a·d = c·b [asta a fost teoria ]
x/7 = 3/4
x = 7·3/4
x = 21/4
x/7 = 3/4 <=> 4·x = 7·3 <=> 4·x = 21 <=> x = 21/4 => x = 5,25
