2. what is the formula for calculating body mass
index? what does it mean for someone to have a
healthy body mass index?

BMI = kg/m2 where kg is a person's weight in kilograms and m2 is their height in metres squared.
A healthy body mass index would be between 18.5 to 24.9.
For someone to have a healthy body mass index it would mean that your BMI wouldn’t be considered underweight or be considered overweight. (Between the two numbers)

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