Final Answer:
Their new grade in math is 80%.
Step-by-step explanation:
If someone has a 100% in math and gets a zero on a project that is 20% of their grade, their new grade in math would be 80%. This is because the project score would bring their overall grade down by 20 points.
Here is the calculation:
New math grade = Original math grade - (Project weight * Original math grade)
New math grade = 100 - (0.2 * 100)
New math grade = 80