5) C

1 birthday each calendar month = 12 x 1 = 12

3 pairs of twins only in April = 3 x 2 = 6

2 pairs of twins not in April = 2 x 2 = 4

We must take into consideration that the two pairs of twins that are not born in April must have been born instead of a single grandchild’s birthday in the other months.

The year COULD be as follows:

January – one of the pair of twins = 2

February – second pair of twins = 2

March – 1

April – three pairs of twins = 6

May – 1

June – 1

July – 1

August – 1

September – 1

October – 1

November – 1

December – 1

This gives us a total of 19 birthdays. However, if we look at the third paragraph, it says “twice as many granddaughters as grandsons”

We cannot divide 19 into a ratio of 1:2

We cannot divide 20 into a ratio of 1:2

However, we can divide 21 into a ratio of 1:2

Hence 21 is the answer. (14 granddaughters and 7 grandsons)

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