Division by a variable, oracle-priced pools, and trading limits split into small swaps

Mentorship Series #12 · 17 December 2025 · @0xfirefist · Mento V3 · Cantina

We continued the Mento V3 contest on Cantina.

Lesson Summary:

Whenever you see a division by a variable, make sure it can never be zero;
The price of FPMM pools comes from the oracle, not from the ratio of the reserves;
Go through FPMM.sol line-by-line;
Work out what rateNumerator and rateDenominator are, and whether they can be manipulated;
How are the fees applied, and do they make sense? If the fee is set to 10%, is 10% actually charged, or more, or less?
Can the trading limits be bypassed? They exist to stop large swaps, so can several small swaps add up to one big one?