'The math is mathing' (or its negative 'the math ain't mathing') means 'the numbers add up' or 'this makes sense' — and the negative means it doesn't. Often used about gossip, conspiracy theories, suspicious bills or relationships that don't quite line up. 'Her timeline ain't mathing' means 'her story doesn't add up'. If your teen says it, they are pointing out something does or doesn't make sense. Playful and harmless.