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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