no need to mention positive integer...the question says find "units digit". by definition, units digits can only be positve integers. so there is no ambiguity.
Can anyone confirm this? I'm not aware of any such rule.
fivedaysleft mentioned a partially correct rule. Digit=0,1,2,3,4,5,6,7,8,9 (So a digit is non-negative one-digit integer)
But, note that y is an Integer, any Integer, NOT necessarily a digit.
Unit's digit of "y" is a digit.
y can be -525324324342424246; Here unit's digit of y=6
y can be +434252424242424244; Here unit's digit of y=4
Please send a mail to MGMAT and get it clarified.