The number of people visiting a park over 1 week was recorded. Determine the interval of the group by dividing the data into 5 groups and calculate the mean of the grouped frequency for the data set: (29, 35, 17, 21, 27, 39, 41}.
Interval of the group:
Mean of the grouped frequency to the nearest whole number:

Each person in an it department of 25 people works on at least one of 3 major projects: system security, program implementation, platform development. 10 people work on system security, 13 work on program implementation, and 12 work on platform development. 4 people work on system security and platform development, 5 people work on program implementation and platform development, and 2 people work on all three projects. how many people work on system security and program implementation?