This chapter includes 39 problems that involves strings, numbers and mathematical operations. The chapter starts with a bunch of classical problems for strings such as counting duplicates, reversing a string, removing whitespaces and so on and forth. The chapter continue with problems dedicated to numbers and mathematical operations such as summing two large numbers and operation overflow, comparing two unsigned numbers, compute the floor of a division and modulus, etc. Each problem is passed through several solutions including Java 8 functional style. Moreover, the chapter covers through problems the futures added in JDK 9, 10, 11 and 12.