[jack jack]

Twice eight are ten of us, yet ten is just three.
Seven of us are five.
Can you now see?
If this is not enough, I will tell you a bit more,
Nineteen of us are eight, and nine is just four.

[rudi_in]

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The riddle refers to the number of letters used in spelling the words.

Twice eight are ten of us,
There are 5 letters in the word 'eight'. Therefore twice 'eight' is 10 (5*2=10)

Yet ten is just three
Three letters in the word 'ten'.

Seven of us are five
Five letters in the word 'seven'.

Can you now see?

If this is not enough, I will tell you a bit more,

Nineteen of us are eight
Eight letters in the word 'nineteen'.

And nine is just four
Four letters in the word 'nine'.