You should need an event wrapped around a coin counter variable.
Compare #Coins >=10
Show the UI with the math question.
Check the answer
If Answer is wrong Goto Layout EndGame
If the Answer is right remove the question UI.
It would be easier to use another layout, but it depends if you game needs to continue or go somewhere else.
With the above, you need to control when to display the math question otherwise as soon as you continue it will show the question again (since #Coins is still > 10)