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1 liter of water equals 1000 milliliters.
To find who has more water, you need to convert (the unit) of each person to either liters or milliliters.
Since it asks for millimeters, I'm going to convert Tutu's 7/10 liters to milliliters. Since there are 1000 milliliters in every liter, you have to multiply 7/10 by 1000. 7/10 multiplied by 1000 is 700.
So Pete has 500 milliliters of water and Tutu has 700 milliliters of water.
Now you need to find the difference of 700 and 500 by subtracting.
700-500=200
Tutu bought 200 more milliliters of water than Pete.
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