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There are many ways to make money in this game but there is a very easy way to make alot of money in one day.
If you take one of your towns native fruit and mail it to one of your towns people you will probably get a fruit that is exotic to your town. If it is not native to your town or is not a coconut, good job with the first step.
Next buy a bunch of axes and chop down every tree in your town. When they are all gone plant the foreign fruit and grow a tree. It is recommended to get more than one foreign fruit just in case one tree dies.
Grow a town full of foreign fruit trees and when they all grow up and harvest all the fruit.
If you have alot of trees multiply the amount of trees by the number of fruit on each tree. Then multiply that by 500. The nimber you get from that math problem would be how much money you would get for all the fruit in your town.