Rafiq exercised for $\frac{3}{6}$ of an hour, while Rohit exercised for $\frac{3}{4}$ of an hour. Who exercised for a longer time?

Rafiq exercised for $\frac{3}{6}$  hour and 

Rohit exercised for $\frac{3}{4}$  hour.

Here comparing the denominator,

The lowest denominator is greater .

$4<6$

Therefore,

$\frac{3}{4}>\frac{3}{6}$

So, Rohit exercised for longer time.