The problem uses the concept of Combination where there are number of members chosen from a group and there order is not important. Thus, the expression that would best describe the given above is 30C6 which means the combination of 30 taken 6. The numerical value for this is 593775.
In April, the water temperature in the top 5 meters ranges between 18°C to 19°C and in August, the temperature at 15 to 20 meters range from 29.5°C at the top to 26°C.