In how many ways can 4 different books be arranged on a shelf so that the red book is next to the blue book?

How many positive integers less than 100 are divisible by both 6 and 9?

If x + 2y = 10 and 3x − y = 9, what is the value of x − y?

A rectangle has area 60 and perimeter 34. What is the length of its diagonal?

Ana runs at 8 mph and Ben runs at 6 mph on a straight road, starting from the same point in the same direction....