The students need to know 1-16.
I work in a small school so I have the kids make pairs or groups. They pick whichever number they want and when its their turn they call it out.
'Three please' and the teacher clicks on the three revealing part of the hidden letter if they are lucky.
Continue until each group has a chance and then they guess what they letter is.
Click on the green bar to reveal the letter.
This can be used quickly without groups as a warm up or quick review of letters and numbers.