# Issue 1, 2017 | OFF AIR

## Saving your ATM System

You have been given 1000 programs to be installed on all the computers of your new ATM system. Each program only takes a few seconds to install. Unfortunately you have been told that one of the programs contains a malware. The malware takes 20 hours to start showing negative effects. Your new ATM system is supposed to go online in 24 hours and you cannot delay the release or risk installing a malware which may infect the entire system! Thankfully you can rapidly purchase some computers which you can use to test which of the programs is deficient.

**What is the smallest number of computers that you will need to purchase to make sure that you identify which of the programs contains the malware?**

The answer will be published in the next edition of ON AIR. Please send your solutions to [email protected] All entries must be received by 30 April 2017. The first correct answer drawn at random after this date will win a pair of Helios noise-cancelling headphones. Good luck!

## And the winner is…

Find the answer to the puzzle "Rogue Drones" below. Congratulations to **Bruce d'Ancey **of Air Safety Support International, who sent the correct answer and came first in the draw.

**ANSWER**

Divide the 12 drones into 3 groups of 4 labeled A, B and C.

**1 ^{st} weighting**: Weight A against B.

If A and B are the same weight:

the mismatched drone is in group C in that case:

**2 ^{nd} weighting**: 3 drones from C (C1, C2, C3) against 3 drones from A or B

If they are all the same weight:

the mismatched drone is the remaining one from group C (C4)

**3 ^{rd} weighting**: Weight the remaining drone C4 against any other to determine whether it is lighter or heavier

If they are not the same weight:

The way the balance skews tells you whether the mismatched drone is heavier or lighter (direction of weight)

**3 ^{rd} weighting**: Weight two of the 3 drones (C1 and C2) for example against each other

If they are the same weight then the mismatched drone is C3 and we know whether it is lighter or heavier.

If they are not the same weight the mismatched drone is the one which follows the direction of weight (lighter or heavier depending on the results of the second weighting)

If A and B are uneven weights:

Make a note of whether A is lighter or heavier than B (direction of weight).

**2 ^{nd} weighting**: Weight 3 from group A and 1 from group B (A1, A2, A3, B1), against the remaining one from group A and 3 from group C (A4, C1, C2, C3).

If they are the same weight:

The mismatched drone is in the remaining from group B (B2, B3, B4).

We know whether it is heavier or lighter based on the direction of weight established before.

**3 ^{rd} weighting**: Weight two of B against each other (B2 and B3).

If they are the same weight then the mismatched drone is B4 and we know whether it is lighter or heavier.

If they are not the same weight the mismatched drone is the one which follows the direction of weight (lighter or heavier depending on the results of the second weighting)

If the direction of weight does not change:

The mismatched drone is in the three As (A1, A2, A3)

We know whether it is heavier or lighter based on the direction of weight established before.

**3 ^{rd} weighting**: Weight two of A against each other (A1 and A2).

If they are the same weight then the mismatched drone is B3 and we know whether it is lighter or heavier.

If they are not the same weight the mismatched drone is the one which follows the direction of weight (lighter or heavier depending on the results of the second weighting)

If the direction of weight changes:

The mismatched drone is either A4 or B1.

We do not know whether it is heavier or lighter.

**3 ^{rd} weighting**: Weight one of them against a normal drone (A1 and C1 for example)

If they are the same weight then the mismatched drone is B1 and we know whether it is lighter or heavier based on the direction of weight of the 1^{st} weighting.

If they are not the same weight then the mismatched drone is A1 and we know if it is heavier or lighter.

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