tell you what
(colloquial) Introduces a compromise or arrangement where the interlocutor has some benefit or advantage.
The best books... are those that tell you what you know already. George Orwell
Tell me what you eat, and I will tell you what you are. Jean Anthelme Brillat-Savarin
If you don't know how to die, don't worry; Nature will tell you what to do on the spot, fully and adequately. She will do this job perfectly for you; don't bother your head about it. Michel de Montaigne
Children will tell you what they do, men what they think and older people what they have seen and heard. Gypsy Proverb
Tell me with whom you go and i'll tell you what you are. Italian Proverb
Tell me what you want to buy and I will tell you what you are. Mexican Proverb