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'Martha's Family Tree' printed from http://nrich.maths.org/
Old Martha has 5 children, each of whom has 4 children, each of whom has 3 children, each of whom is childless.
How many descendants does Old Martha have?
If you liked this problem, here is an NRICH task
which challenges you to use similar mathematical ideas.
This problem is taken from the UKMT Mathematical Challenges.