That is called doing the inverse. Here is doing the inverse. You do the inverse to check if it's correct.
1. 30÷10=? You start off with an number question.
2. 30÷10=3 Answer the number question.
3. Swap around the two end numbers(this only works with multiplication and division). 30÷10=3 => 3÷10=30
4. At the moment it doesn't look right, but once you change the x to ÷. 3x10=30
You did that.
Note: To type ÷ on a computer you have to hold alt and type 0247 on the number pad at the same time.