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Target Practice Plus #17

“Uncle Bob”

Here’s a new twist on a familiar activity: using simple arithmetic, construct a target number from a given set of numbers. For example, let’s construct 135 from the set

{2, 3, 4, 15, 50, 100}

We must use only the four operations: addition, subtraction, multiplication, and/or division. Numbers in the set can be used only once in the solution. One solution is

(2 + 3 + 4) x 15 = 135

You can see that using grouping symbols is also permitted, or in other words, the final result can be made from preliminary ones, as in

2 + 3 + 4 = 9
9 x 15 = 135.

Here’s the twist. The solution above uses four of the six given numbers. Can we find a more concise solution? I find that this activity requires me to be creative and flexible in my thinking. We'd like a second solution, if it exists, to use fewer than four members of the set. I found one using three numbers, and you may have too:

100 + 50 – 15 = 135

Try these three challenges. I rank them as easy, medium, and hard in difficulty.

1. Make 15 from {4, 9, 20, 48, 62, 100}. There are solutions using 4 or 5 of the set.

2. Make153 from {2, 7, 11, 19, 35, 75}. There are solutions using 4 or 5 of the set.

3. Mak 279 from {4, 5, 13, 15, 35, 50}. My solutions required all six numbers. Can you find a more economical one?

solutions

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