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We make the fractions improper so:
9 \frac{3}{4} = \frac{39}{4}  \\ 15 \frac{2}{4} =15 \frac{1}{2}=\frac{31}{2}

Their rates is the reciprocal of the time they take so the difference is:
 \frac{4}{39} - \frac{2}{31} = \frac{124-78}{1209} = \frac{46}{1209}

Therefore the first train is  \frac{46}{1209} units per hour faster than the second