Please Solve this :-

a) 36:81: :x :63

b) 27 : x :: 63 : 84

c) 25 : 35 : : 35 : x

(a).  36 : 81 : : x : 63
Since the given numbers are in proportion, then
product of the means = product of the extremes

$$\begin{gathered} \Rightarrow 81 \times x = 36 \times 63 \\ \Rightarrow x = \frac{36 \times 63}{81} = 28 \end{gathered}$$

(b).  27 : x : : 63 : 84
Since the given numbers are in proportion, then
product of the means = product of the extremes

$$\begin{gathered} \Rightarrow x \times 63 = 27 \times 84 \\ \Rightarrow x = \frac{27 \times 84}{63} = 36 \end{gathered}$$

(c).  25 : 35 : : 35 : x
Since the given numbers are in proportion, then
product of the extremes = product of the means

$$\begin{gathered} \Rightarrow 25 \times x = 35 \times 35 \\ \Rightarrow x =\frac{35 \times 35}{25} =49 \end{gathered}$$