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How do you determine whether a relationship represented as as graph is linear or nonlinear

Alice is paying her bill at a restaurant. but tax on the cost of a male is 5%. she decides to leave a tip 20% of the cost of the meal plus the tax

In 1995 america online had about 3000000 users over the next decade users are expected to increase from a few million to the tens of millions suppose the number of users increased by 150% by the year 2000 how many users will there be in the year 2000