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 10 Two Trains Two trains (Train A and B) with 200 km between them are travelling towards each other at 100km/h. A mosquito, initially standing on the front of Train A, starts travelling towards Train B at 150km/h. O ... Submitted by Ravi Doable 4.5 9 The Monty Hall Problem You’re on a game show, and you’re given the choice of three doors: Behind one door is a car and behind the other two doors are goats. You pick a door, say No. 1, and the host, who knows what’s behind ... Submitted by Ravi Doable 5.5 15 Swap Variables in Place without XOR Suppose you have two variables, a and b. How can you swap their values without using a temporary variable and also without using the bitwise operator XOR? Submitted by Valentin Doable 3.9 37 No if() Only abs() Given two variable a and b, construct a function f(a,b) which return the maximum one in {a,b}. You can only use +- · / and abs() (to take the absolute value). NO if() is allowed in your construction. ... Submitted by MoralMachine Doable 5.8 11 Gold or silver? What weighs more, a pound of silver or a pound of gold? Submitted by Jango Easy 2.0 10 Sum of Two Prime Numbers If p, q > 2 are consecutive in set of primes. Since p,q can only be odd number, (p+q) is an even number. Can (p+q)/2 be prime? from http://republicofmath.com/ Submitted by MoralMachine Easy 3.2 8 Non-Adaptive 12 Identical Balls Okay - so I posted this puzzle to reddit a few weeks ago and one guy had the following comment: Same problem, but your solution must be non-adaptive. IOW, specify the weighings that you will make up f ... Submitted by Aakash Hard 9.0 6 Grey Squares Illusion Square A is the exact same shade of gray as square B. It's true, check out the reconstruction. Submitted by Ravi Doable 6.0 17 Multiples of 8 plus 1 Squaring the first several odd numbers reveals the following pattern: 3 squared = 8 + 1 5 squared = 24 + 1 7 squared = 48 + 1 8, 24, and 48 are all multiples of 8. Does this pattern hold for all squa ... Submitted by Heather Moore Doable 4.5 21 Odd Pills Your pharmacist gives you 5 jars filled with pills. The pills contained in each jar have an identical appearance and size (i.e. if you mixed all of the pills you wouldn't be able to tell them apart). ... Submitted by Jay Elliott Doable 3.6 6 Partitioning a set In how many ways can a set of r objects be partitioned into subsets of k objects such that: all subsets are mutually exclusive each of k-object subset appears in exactly one of the partitions [Partit ... Hard 8.7 5 Count The Bits Write a function that counts the number of bits in an integer that are set to 1 that is linear in time complexity in the worst case, but sub-linear on average. Submitted by Jay Elliott Hard 7.6 5 Matchsticks This is a classic Martin Gardner puzzle. See the picture - the four matchsticks represent a martini glass and the match head represents an olive. Move just two matches so that the glass is reformed bu ... Submitted by Aakash Doable 3.6 9 1978 The year 1978 is such that the sum of the first two digits and the latter two digits is equal to the middle two digits, i.e. 19 + 78 = 97. What is the next year (after 1978) for which this is true? Submitted by Ryan Miller Doable 6.6 46 Charlie and the Chocolate Factory Charlie once visited chocolate factory to buy some chocolates.Chocolate cost Rs. 3 each and he had only Rs. 45.He was disappointed. He wanted more. But when he reached the factory he was amazed. There ... Submitted by Mikit Kanakia Easy 2.4