If the crankshaft of a two stroke engine is rotating at 900 rpm how many power strokes occur each second?

The crankshaft of a two-stroke engine completes one revolution for each power stroke. So we can divide the RPM by 2 to get the number of power strokes per second.

$$Power \space strokes \space per \space second=\frac{900 \space rpm}{2}=450 \space strokes/s$$