Question

How would the tides on earth be different if the moon revolved around the earth in 15 days instead of 30

Answer 1

In case that this happened, and nothing else changed like the rotational speed or anything similar, everything would remain the same, just the time passed between two tides would change. Instead of the regular spring and neap cycle, you would have it reduced by half, proportional to the reduction of time. If other things regarding this changed too then there would be more changes.

Answer:

If the moon revolved around the earth in 15 days instead of 30 days then there would be less time between each tide. They would come much faster. 

Explanation:

Neap Tides are those in which the difference between high and low tide is the least. They occur twice a month when the Sun and Moon are at right angles to the Earth. When this is the case, their total gravitational pull on the Earth's water is weakened because it comes from two different directions. On the other hand, spring tide are those in which the difference between high and low tide is the greatest. Spring tides occur when the Moon is either new or full, and the Sun, the Moon, and the Earth are aligned.

But if the moon moved around the earth faster, while the earth maintained the same rotational speed, the period between spring and neap tides would be less.

Answer 2

-- The low tides would be lower, and the high tides would be higher.

-- The time from a low tide to a high tide, or from a high tide to a low tide,
would average 6hrs 24min, instead of the present  6hrs 12.2min.

(By the way ... the moon revolves around the Earth in 27.32 days.
That "30 days" that you mentioned is close to the complete cycle
of the moon's phases ... 29.53 days ... which the tides also follow.)