## Plants to Water

I have two large plants to water. A liter of water lasts 14 days if I use it on the first plant. It lasts 10 days if I use it on the second plant. How long would it last if I used it for both plants?

## Calendar Square of Dates: Formula

Take a calendar, and draw a square around any nine dates (it will be a 3 x 3 square, right?) Let s be the smallest number in the square. If you add up all the numbers in the square, the answer will always be a multiple of 9. In particular, the sum will be exactly 9 · (s + 8). Why?

## Four 4’s: 21 – 40

In a previous problem, you found mathematical expressions equalling each of the numbers 1 – 20 using exactly four 4’s and any mathematical symbols you wanted to. Now make 21 – 40 following the same rules.

## Horses and Harness

Jessica bought two horses and a harness for a total of \$4500. The value of the better horse and the harness together was twice the value of the poorer horse. What is the value of the poorer horse? (You don’t need algebra for this: do it in your head!)

## Heap, Egyptian Equation

What may be the earliest note of an equation is found in the ancient Egyptian records of Ahmes in the following form: “heap, its two-thirds, its half, its seventh, its whole gives 97.” If we interpret “its two-thirds” as “two thirds of the heap” and assume that we are adding all of these amounts, we can find out how big the heap is. Proceed.