Combining four 9s to obtain different numbers ?

 Question. Can I use +,-, /, * and four 9s to obtain 100 ?

Answer. 99 + 9/9 -> 99 + 1 -> 100

 Question. Can I use +,-, /, * and four 9s to obtain 111 ?

Answer. 999 / 9 -> 111

Explaining the concept of a fraction

 So, what does X/Y mean ?

One way to think about X/Y is to think of as a two step mathematical operation.

Step 1. Divide an item ( any item ) into Y equal pieces. 

Step 2. Of those Y equal pieces, pick X pieces.

I find that this divide and pick explanation is one that is easy to get across to children.