8.4 ÷ 4 = 2.1
Let us assume the missing fraction be x,
Then,
8.4 ÷ x = 2.1
8.4/x = 2.1
By cross multiplication, we get,
x = 8.4/2.1
x = 84/21 … [divide both numerator and denominator by 3]
x = 28/9 … [divide both numerator and denominator by 9]
x = 4