Minimum space required?
Krishna wants to grow rose, orchid, and jasmine trees in his garden. Planting n rose trees requires square meters, planting n orchid trees requires 5n square meters, and planting n jasmine trees requires square meters. If she plants 10 trees, what is the minimum amount of space that her garden will occupy (in square meters)?
The answer is 40.
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Starting from 0 trees, add the next tree that will add the least cost.
Orchid trees always cost 5 sqm per tree: 5, 5, 5, ...
Rose trees cost more and more: 1, 3, 5, 7, 9, ...
Jasmine trees cost more and more: 1, 7, 19, ...
Viewed in this way, rose trees and jasmine trees should not be planted if they will cost more than 5 sqm, and should be planted if they cost less than 5 sqm.
Hence, we should plant 1 jasmine tree, 2 rose trees and 7 orchid trees.