4,294,967,296
I think...
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56,756,000,000,000,000
10 followed by 16 zeros: 10,000,000,000,000,000 100000000000000000
Power 2: units digit 9. Multiply by 49 again to get power 4: units digit 1. So every 4th power gives units digit 1. So 16th power has units digit 1, so the previous power, the 15th must have units digit 3.
the answer is 4
4 to the 20 power is 1099511627776