The United States Postal Service (USPS) sells money orders identified by an 11-digit number x1x2 . . . x11. The first ten digits identify the money order; x11 is a check digit that satisfies x11 = x1 + x2 + . . . + x10 mod 9.

(b) [3 points] Find the check digit for the USPS money orders that have an identification number that start with these ten digits 3289744134.

(c) [3 points] Determine whether 66606631178 is a valid USPS money order identification number