For calculating the units digit ,you only take last digit in each number and start multiplying them and when ever a 2 digit number is generated,you take only the units digit of that as well and go on.
Therefore 13*27*63*51*98*46 = 3*7*3*1*8*6 = 21*3*1*8*6 = 1*3*1*8*6 = 24*6 = 4*6 = 24 = 4
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