Answer: Garrick's plant is shorter than Zan's plant.
Each millimeter of error results in a 11.11% percent error.
Garrick's measurement was 2 millimeters off and Zan's was 12 millimeters off.
Garrick's measurement was 20% off and Zan's was 12% off.
Explanation:
Since we have given that
In case of Garrick's plant:
Actual height of his plant = 10 mm
Measured height of his plant = 8 mm
In case of Zan's plant :
Actual height of his plant = 100 mm
Measured height of his plant = 88 mm
So, clearly Garrick's plant is shorter than Zan's plant.
Difference in actual height of plants of Garrick and Zan is

similarly, difference in measured heights of plants of Garrick and Zan is

Percent error will be

So, Each millimeter of error results in a 11.11% percent error.
Error in Garrick's plant :

Percent error is given by

Similarly,
Error in Zan's plant ;

Percent error is given by

Garrick's measurement was 2 millimeters off and Zan's was 12 millimeters off.
So, Garrick's measurement was 20% off and Zan's was 12% off.