20 flowers = 4.2 $ = 420¢
20/5=4 flowers
420/5=84¢
4 flowers = 84¢=0.84$
$0.84
20/4.2 = 4/x
Use cross multiplication,
20/4.2 = 4/x
=> 4.2(4) = 20x
=> 16.8 = 20x
Reverse,
16.8 = 20x
=> 20x = 16.8
Simplify and change it to decimal numbers,
20x = 16.8
=> x = 16.8/20
=> x = 168/200
=> x = 0.84
4 flowers cost $0.84