Harry scores a mark of 64 out of 85 for mechanics and 69 out of 91 for real analysis. By finding the LCM, state which examination was a better performance for Harry.
The LCM of 85 and 91 is 7735.
The mechanics score is
$$\frac{64}{85} = \frac{5824}{7735}$$
The real analysis score is
$$\frac{69}{91} = \frac{5865}{7735}$$
The better performance was in real analysis.