Calculate the missing quantities of the proportions.

a) [[$ \displaystyle\frac {2} {14} = \displaystyle\frac { } {21} $]]



The missing quantity is .

b) [[$ \displaystyle\frac {4} { } = \displaystyle\frac {8} {6} $]]



The missing quantity is .

c) [[$ 2 : 8 = 3 : $]]



The missing quantity is .

d) [[$ : 5 = 4 : 8 $]]



The missing quantity is .