number puzzles

1. Kenken
   
   In Kenken, you need to place the counting numbers in the squares just like in Sudoku, but additionally, you are given boxes with clues with math operations. The numbers you place in each box have to make the "clue" number using the given operation.
   So, Kenken is an excellent game to practice the four operations and logical thinking. Lower elementary children can use addition/subtraction and others can use multiplication/division or all four operations.
   Play Kenken online here. You can choose the size of the game, operations, and the difficulty level.
2. Numbers in a triangle
   
   
   
   Put the numbers from 1 to 9 in the circles so that the sum of the numbers on each side of a triangle is the same.
   
   At CoolMath4Kids you can view a hint and the solution.
   
3. Kakuro
   
   Kakuro puzzles are "cross-sum" puzzles—like mathematical crossword puzzles. Each "word" must add up to the number provided in the clue above it or to the left.
   Download 5x5 Kakuro puzzles at KrazyDad. Other sizes exist also.
4. Same number - or twice as many
   
   Here is a puzzle that seems simple, but is a good critical thinking challenge for lower elementary school kids. You can easily reword it for example to have two flocks of birds in two trees. 
   Farmer Brown and Farmer Green were ruminating one day on the fence between their farms. Farmer Brown says, "You know I was just thinking. If you gave me one of your cows, then we would have the same number of cows." Farmer Green replies, "If you gave me one of your cows, then I would have twice as many as you!"
   
   How many cows does Farmer Brown have and how many cows does Farmer Green have?
   
5. More than two animals
   
   This puzzle is from Grace Church School's collection of Abacus problems 2013-2014 for grades 3-8. The site does not supply the answers, but students are encouraged to submit their answers and they will receive a reply. 
   I have more than two animals at home. All of them are dogs, except for two. All of them are cats, except for two. All of them are hamsters, except for two. What kinds of animals and how many of each animal do I have?
   
6. Brick's weight
   
   This is well-known problem that sounds so simple, yet it fools many people!
   A brick weighs one kilogram plus half of the brick. What is the weight of one brick?
   
7. Average Miles per Hour
   A car is going up a hill. The hill is one mile long. The driver goes up the hill at an average speed of 30 miles per hour. When the driver reaches the top of the hill, he starts down the other side. The downhill side is also one mile long. How fast must the driver go down the hill in order to average 60 miles per hour?
   
8. Legs in the bus
   1. There are 7 girls on a bus.
   2. Each girl has 7 backpacks.
   3. In each backpack, there are 7 big cats.
   4. For every big cat there are 7 little cats.
   
   How many legs are on the bus, not counting the driver?
   
9. How can these be equal?
   
   Here's a short but puzzling puzzle that someone sent for the contest:
   In which meaning 1070 = 1110 ?
   
10. Coconut trader
    An intelligent trader travels from 1 place to another carrying 3 sacks having 30 coconuts each.No sack can hold more than 30 coconuts. On the way he passes through 30 checkpoints and on each checkpoint he has to give 1 coconut for each sack he is carrying. How many coconuts are left in the end?
    
11. Milkman's Containers
    A milkman has an 8-liter container full of milk, and also two empty containers that measure 5 liters and 3 liters. He needs to deliver 4 liters of milk to a customer.
    
    The milkman has no other spare containers and no way to mark any containers. He does not want to pour milk away. How will he measure the 4 liters of milk?
    
12. Farmer and his ducks
    
    A farmer was asked how many ducks he had. "Well," he said, "they ran down the path just now and I saw one duck in front of two ducks, a duck behind two ducks, and a duck between two ducks." How many ducks were there?
    
13. Number puzzles with many operations
    
    Enter each of the numbers from 1 to 9 in the squares, each one only once. The order of operations doesn't apply! These puzzles are available in four different levels of difficulty.