News that the U.S. Treasury Department would be replacing Alexander Hamilton on the $10 bill with a woman left many wondering why Andrew Jackson's face on the $20 bill wasn't being replaced instead. But Treasury Secretary Jack Lew says there's a straightforward explanation.

The $10 bill is currently scheduled for a redesign, and creating new currency is a painstaking process that takes years to plan and complete.

The $10 is the first in a planned new series of redesigned notes, Lew said on a press call Wednesday. It's a process that began before he took office, he said, and that he has been discussing with the Treasury since the beginning of his tenure in 2013. The new bill won't be unveiled until 2020.

It is a "happy coincidence" that the timing of the decision occurred soon after outside, private campaigns to push for replacing Jackson with a woman, he said.

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