_AHA! Puzzle This By Michael H. Pryor

MAKE’s favorite puzzles. (When you’re ready to check your answers, visit makezine.com/14/aha.)

River Crossing

A dysfunctional family has to cross the river. On one side of the river are the mom and 2 daughters, dad and 2 sons, the maid, and the dog. There is a boat only big enough to hold 2 people (counting the dog as 1 person). Only the adults (mom, dad, maid) are capable of operating the boat. Everyone has to get to the other side, without anything bad happening.

There are some difficulties. If the dog is left with anyone and the maid isn’t there to control him, he’ll bite. The dad can’t stay with any of the daughters when the mom isn’t there, or they’ll get into an argument. Likewise, the mom can’t be alone with either of the sons when the father isn’t there.

Remember, only an adult can operate the boat (and it can’t drive itself). How do they cross?

Cheesecake

An obsessive-compulsive ant starts at the top left-hand corner of a square, checkered tablecloth. He’s trying to reach the bottom right corner with the cheesecake in it. He will only move at right angles following the grid lines of the tablecloth. Also, he will only move toward the cheesecake (in other words, down or to the right, never up or to the left).

If the tablecloth is a 2× 2 square, the ant has 6 distinct paths he can take (see illustration). How many paths are there if he’s on an 8× 8 checkered tablecloth?

Illustrations by Roy Doty

Michael Pryor is the co-founder and president of Fog Creek Software. He runs a technical interview site at techinterview.org.

174 Make: Volume 14