Question

121 : 21 :: 100 : ?

Answer 1

121/21=100/x
121x=2100
x=2100/121
x=17.35

Answer 2

Ratio and Proportion

121:21::100:?

so above term can be written as follow,

121:21::100:x

121/21=100/x

":" is used as a divide symbol or we can say it is used for division purpose and "::" is used as equal to. ":" is called as ratio or division.

and "::" is called as proportion.

5.761=100/x

now by cross multiplication

x=17.358

the value of x is 17.358