For #28, could you explain in more details? I don't understand what the following means:

"Let the sum of the digits of a be 11x. Then the sum of the digits of b will be 11x + 1 + 1 –

9y = 11x + 2 – 9y (y is the number of carries of the sum of a and 11)."

For #29, I think you forgot to state that when both Alex and Bob increase their speed by 2km per hour, they'll meet in 6 hours.