A Kuthean — a Samaritan — once came to Rabbi Meir with an accusation against the patriarch Jacob. It is preserved as exemplum No. 32 in Moses Gaster's 1924 collection.
"Your ancestor was not so righteous," the Samaritan said. "When Jacob vowed to give God a tenth of all that was his (Genesis 28:22), he set aside only Levi from his children — one out of ten tribes. But count carefully. Joseph had two sons, Ephraim and Manasseh, and each became a tribe (Genesis 48:5). That makes fourteen sons altogether. A proper tithe would be one for every ten, which means Jacob should have separated a second, even a third. He short-changed his tithe."
Rabbi Meir answered without raising his voice. "You are counting wrong. There were four mothers — Leah, Rachel, Bilhah, and Zilpah — and each of them bore a firstborn. The firstborn, according to Exodus 13:2, are sacred to the Holy One; they belong to Him already, before any tithe. Subtract the four firstborn from the fourteen tribes and what remains? Ten. And one tenth of ten is Levi."
The Samaritan had no reply. The math was correct; more than that, it was deeply Jewish math — math that counts what is already given to God before it counts what must be given. This is how the rabbis read Torah. Before you calculate your obligations, remember what was never yours.