Sisters and brothers
Problem
Cassandra, David and Lachlan are brothers and sister.
They range in age between $1$ year and $14$ years.
Can you figure out their exact ages from the clues?
Lachlan's age and David's age added together is $10$ years.
Cassandra's age and Lachlan's age added together is $15$ years.
David's age and Cassandra's age added together is $19$ years.
This problem was made up and sent in by Cassandra from Cambridge in the UK. Thank you Cassandra!
Getting Started
What does that tell you about the ages of the other children?
What do you know about David's age compared with Lachlan's age?
Student Solutions
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.