There are 8 balls. 7 of them weigh the same. 1 of them has a different weight, (you don't know if it's heavier or lighter). How do you find the odd ball with 2 weighs?
How many 4 digit numbers can be formed by using 0, 1, 2, 3, 4, 5, 6, and 7? How many of them are between 4000 and 6000?