A train covers 450 km at a uniform speed. If the speed had been 5 km/h more, it would have taken 1 hour less to cover the same distance. How much time will it take to cover 315 km at its usual speed?

Let the uniform speed of train = s

According to the problem,

$$\frac{450}{s+5}=\frac{450}{s}-1$$

$$\frac{450}{s}-\frac{450}{s+5}=1$$

$$450\left(\frac{s+5-s}{s\left(s+5\right)}\right)=1$$

$$450\left(\frac{5}{s^2+5s}\right)=1$$

$$s^2+5s-2250=0$$

$$\left(s+50\right)\left(s-45\right)=0$$

s = -50 or s = 45

s cannot be negative, so s = 45 km/h

The uniform speed of train = 45 km/h

Time taken by train to cover 315 km at its usual speed = $$\frac{315}{45}$$ = 7 hours

Hence, the correct answer is Option D