Have another one that I need done ASAP please and thank u!!!

What's the difference between: x≥0 and nonnegative integer. ?

Amotorboat maintained a constant speed of 12 miles per hour relative to the water in going 45 miles upstream and then returning. the total time for the trip was 8.0 hours. use this information to find the speed of the current.

Asingle die is rolled twice. find the probability of rolling a 3 the first time and a 2 the second time