It will be:

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(10k / (a / 2)) = b

10k * b = c

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(BALLS / (a / 2)) = d

BALLS * d = e

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The total supply of the merged chain will be a, or c + e. The exact ratio of the swap will be unknown until deposits are closed. It's not going to be a flat rate; half the coins on the new chain are going to BALLS and half are going to 10k, regardless of the amount that are turned in. If less people swap over, each piece of the pie is worth more

EVERY SINGLE COIN ON THE NEW CHAIN WILL BE SWAPPED AT ONE TIME. THERE ARE NO EXTRA COINS AT ALL