If 6, 18, 39, and x are in proportion, then find the value of x.

Given: 

6, 18, 39 and x are in proportion.

Concept Used: 

If a, b, c, and d are in proportion then 

​$$a : b :: c : d \implies \frac{a}{b} = \frac{c}{d}$$ 

Solution: 

​$$6: 18:: 39:x \\ \ \\ \implies \frac{6}{18} = \frac{39}{x} \\ \ \\ 6x = 39 \times 18 \\ \ \\ x = \frac{ 39 \times 18}{6} \\ \ \\ \bf x = 117$$​​​​