Question

2:x = x:50. what one number can replace x?

Answer 1

$2:x=x:50\to\dfrac{2}{x}=\dfrac{x}{50}\ \ \ \ |cross\ multiply\\\\x\cdot x=2\cdot50\\\\x^2=100\to x=\pm\sqrt{100}\\\\x=-10\ \vee\ x=10$

Answer: -10 or 10.

Check:
2:(-10) = -0.2
-10:50=-0.2

2:10=0.2
10:50=0.2

Answer 2

X = 10
2:10=10:50 = 1:5