Hello there!
Your questions states: The ratio of the ages of two children is 2:3. If the younger child is 4 years old, how old is the other child?
We know when child A is 2, child B is 3.
To make child A 4, we must multiply his age by 2.
Now we must multiply child B's age by 2, as what you to one side, you must do to the other.
2 * 3 = 6
When child a (the younger child) is 4, child b is 6.
Hope I helped!
Let me know if you need anything else!
~ Zoe
