So you understand the physical fact that uranium239 decays to lead, and the fact that potassium-40 decays to argon and calcium.
Got it.
But you refuse to understand that both do so at constant rates. That's your problem right there.
"The Christian church, in its attitude toward science, shows the mind of a more or less enlightened man of the Thirteenth Century. It no longer believes that the earth is flat, but it is still convinced that prayer can cure after medicine fails." H L Mencken