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Place this "worm" on the 100 square and find the total of the four squares it covers. Keeping its head in the same place, what other totals can you make?

Sisters and Brothers

Age 7 to 11 Challenge Level:

We received this solution from someone who didn't give their name:

Lachlan is $3$
David is $7$
C is $12$

I first looked at the ones with Cassandra in and found the difference, which was $4$, so that means David and Lachlan's ages have a difference of $4$. I then worked out that $7$ and $3$ have a difference of $4$ and make $10$. So, either David is $7$ and Lachlan is $3$ or the other way round.

When I took $7$ from $19$ to get Cassandra's age, that made $12$ and it also made $12$ if I took $3$ from $15$, so it must be that way round.