Math and logic puzzles

My publications tend to include few math and logic puzzles, in part because I have little patience for them, and in part because I often can’t properly illustrate them when they involve pennies or matchsticks, for example. However, here’s one that is easy to show:

5 + 5 + 5 = 550

Can you add one straight line to make the equation accurate? (Putting a line through the = sign is clever, but not what we’re looking for.) Think about it a bit before looking up the answer, which is simply this: Draw a diagonal line on one of the plus signs to make it a 4. Logical thinkers may figure out the answer quickly, but if others are frustrated by it, use it as a chance to show how we each bring unique skills to any group.

Here’s another one to solve that actually depends on words: Look at the following sequence and note that all the numbers from 1 to 15 are shown except 4 and 9. Where do they belong in the sequence and why?

8, 11, 15, 5, 14, 1, 7, 6, 10, 13, 3,12, 2

Like the previous exercise, this one is good for filling in time while people are regrouping; offer a small prize to anyone who comes up with the solution. The answer lies not in the numbers but in their spelling: They are in alphabetical order.
(Adapted from 101 More Games for Trainers by Bob Pike)