Greetings to All!

This is my very first post on a Math forum. I am studying for GRE and there's one particular problem type on ratios which confuses me totally.. Multiple ratios combining..

Say I have two ratios with me

A : B => 21:23

C : D => 9:7

How do I determine A : B : C : D?

Thx a lot for your replies and your time..

Sincerely,

Abhijit.