What are some good problem books, problem solving workshops?

Question

Discrete Math

Good afternoon! Below are a couple of tasks that puzzled me. Tell me, what good problem books, workshops exist, where the methodology for solving such problems is considered. In general, any good literature on solving problems in the field of discrete mathematics, graph theory, probability theory, and other near-programming disciplines would be useful. Thank you!
Problems:
1. It is known that for all positive integers k the number 99^(22k) + 100^(22k) - 2 is divisible by some prime common to all k. For what?
2. Find the sum of all 4-digit numbers that can be made up of the numbers 1,2,4,5. using at least two different numbers.

Answer 1

1) According to Terver, the best textbook for beginners is definitely Feller - Introduction to probability theory and its applications, search here gen.lib.rus.ec
2) Numerical methods - many many things - I liked Bakhvalov
3) Discrete mathematics - try Redkin
Tasks:
1) is divisible by 23
2) did not understand the condition :-)

Answer 2

Find the sum of all 4-digit numbers that can be formed from the numbers 1,2,4,5. using at least two different discrete digits
. n to k