Final answer:
The maximum thickness the key can be and still fit is 7.05 mm.
Step-by-step explanation:
To find the maximum thickness the key can be and still fit, we need to calculate the difference between the ideal thickness of 7.3 mm and the maximum allowable difference of 0.25 mm. To do this, we subtract 0.25 mm from 7.3 mm: 7.3 mm - 0.25 mm = 7.05 mm. Therefore, the maximum thickness the key can be and still fit is 7.05 mm.