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84.42 or 42.21
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 digit in the units column of the number 7157 is 7.
the unit digit is the last digit from left or ones digit
There is no simple formula for that; I suggest you look up the prime numbers in a table of prime numbers, analyzing each prime number in the range 10-99 whether it fulfills this specific criterion.