if last character is 1 or 2 or 3 or 4 or 5 make it 0
else if ( c [ n - 1 ] == '1' c [ n - 1 ] == '2' c [ n - 1 ] == '3' c [ n - 1 ] == '4' c [ n - 1 ] == '5' ) { c [ n - 1 ] = '0' ; return c ; } else { c [ n - 1 ] = '0' ;