Bus tickets in a certain city contain four numbers, U, V , W, and X. Each of

Question:

Bus tickets in a certain city contain four numbers, U, V , W, and X. Each of these numbers is equally likely to be any of the 10 digits 0, 1, . . . , 9, and the four numbers are chosen independently. A bus rider is said to be lucky if U + V =W + X. What proportions of the riders are lucky?