Question

121 : 21 :: 100 : ? what is the number in question mark ?

Answer 1

121 : 21 :: 100 : ?  

The right answer is 121:21::100:19

Given equation  

It is very simple and very interesting  

We can solve this equation by square root

121= square of 11 that is (11*11)=(121)

11*2=22-1=21

100= square of 10 that is (10*10)=(100)

10*2=20-1=19

The  equation is become 121:21::100:19

Answer 2

Answer:121:21::100:17.35

Solution:

Let that number is x

So we can write it as

121:21::100:x

it proportionality can be solve by, multiplying outer terms and equating with multiplication of internal terms

$121x = 21 \times 100 \\ \\ x = \frac{21 \times 100}{121} \\ \\ x = 17.35$
so, that proportion holds true only if ? = 17.35

Hope it helps you.