1. On the main menu, press the small 'burger' button to the right.
2. A number of new buttons will appear, press the one with an 'i' on it.
3. A screen will then appear that has a 'Restore Purchases' button.
4. Press the 'Restore Purchases' button and follow any instructions that pop-up.
If that doesn't work, please make sure you're signing into the same account you used to purchase the questions in the first case.