Answer: This is as easy as it looks so i will try my best to help you.
Step-by-step explanation:
ok so if the object itself is 5mm long but it looks 10 cm long under a microscope you first need to find out how many millimeters 10 centimeters is first
10 cm= 100 millimeters
Now that you know the original object was 5mm but looks 100 mm under a microscope you can divide 100mm/5mm.
100mm/5mm=20
So you know it was a magnified by 20.
Hope that's right
If it is your welcome if it isn't then good luck