In a day, there are 24 hours. So individual spend 5/12 of a day in amusement park means that 5/12 of 24 hours in amusement park.
Determine the number of hours individual spent in amusement park.
Individual spend 2/5 of the time is riding water.
Determine 2/5 of 10 hours to determine the time spend by individual for riding water.
So individual spend 4 hours on riding water slides.