This seems like PRECISELY the kind of situation where the lies would be discovered because they would have the WRONG UNDERLYING MATH SOMEHOW. Tell them NOT to.
Keltham could of course ask questions like these they'd all have no choice but to lie about, but hopefully first they can learn a bit more of the Law...
She puts down 1/3. The rest of her group puts it lower: 1/8, 1/4, 1/4, 1/10, 1/20.
The other group has 4/5, 1/4, 1/2, 1/3, 1/2.