A Step-by-Step Walk-through a Decision Making Puzzle


This is a UCAT guide from UniAdmissions, they provide courses to prepare for the University Clinical Aptitude Test (UCAT) and provide support for entrance into competitive courses like Medicine and Dentistry.

Logical puzzles are one type of question that you can come across in the Decision Making subtest (for other Decision Making question-types read more). These types of questions require you to make an inference based on the available information to get to an answer. 

It will usually be presented with a general statement, than more specific statements. Both should be combined to make a conclusion and find the answer.

The question we will look at is an interesting one.

Decision Making Puzzle Question

Step 1

As always, begin by reading the question “What is the minimum number of grandchildren they have?”

Step 2

The actual information begins at “there is one birthday per calendar month” as the other information is unnecessary. This is where you can begin to formulate an answer. As there are 12 months in a year, you can deduce that the couple has at least 12 grandchildren.

Step 3

The next piece of information is that April sees the birthdays of 3/5 pairs of twins.

This is where drawing a quick diagram will begin to come in use although you can do the calculations in your head. This is the diagram we tend to use when looking at a question like this, but it is by no means the only method;

Decision Making puzzle solution

The letters represent months of the year, the blue lines represent a grandchild. Since there is a birthday every month, we can put one blue line in each month. Birthdays of three pairs of twins occur in April, so six total lines can be added to April (one for each child). There are two remaining pairs of twins, but as the question doesn’t state when their birthdays are, we can put them anywhere. We chose to put them in January and February.

This gives us a total of 19 grandchildren so far. Let’s move onto the next piece of information.

Step 4

“They have exactly twice as many granddaughters as grandsons.”

This is a tricky piece of information to use correctly. The way to use it is to consider that the total number of grandchildren must be divisible by 3. For example, if the couple has 3 grandsons, then they have 6 granddaughters, bringing the total to 9.

8 grandsons would mean 16 granddaughters and 24 total grandchildren – still

divisible by 3.

Step 5

The question asks “what is the minimum amount of grandchildren?”.

Using the information from our diagram, we have at least 19 grandchildren. We need to find the next number, which is divisible by 3.

The next multiple of 3 is 21. Therefore we can deduce that the minimum amount of grandchildren the couple have is 21.

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