<human>:  Mitch has saved $20000 to finance his boating hobby. A new boat costs $1500 per foot in length. If Mitch needs to keep $500 for a license and registration, and three times that amount for docking fees, how many feet is the longest boat he can buy?
<bot>: How much money does Mitch need to keep for docking fees? ** Mitch needs to keep 500 * 3 = <<500*3=1500>>1500 for docking fees.
How much money does Mitch need to keep? ** Thus, he needs to keep 500 + 3 * 500 = $<<500+3*500=2000>>2000 for license, registration, and docking fees.
How much money does Mitch have left to spend? ** He has 20000 - 2000 = $<<20000-2000=18000>>18000 left to spend on a boat.
How many feet is the longest boat he can buy? ** Therefore, the longest boat he can buy is 18000 / 1500 = <<18000/1500=12>>12 feet long.
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