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Day of the Triffids

Age 14 to 16 Short
Challenge Level

Jasmine spends exactly £120 on three types of plants.

Poison ivy plants cost £2 each.
Deadly nightshade plants cost £9 each.
Triffids plants cost £12 each.

She buys twenty plants in total, including at least one of each type.

How many triffids did she buy?
 

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.
You can find more short problems, arranged by curriculum topic, in our short problems collection.