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.