At the beginning of the school year, there were three 4th grade classes. mr. smith’s class had 19 children, mr. day’s class has 17 children, and ms. brown’s class had 20 children. at the end of the year there were still three classes, but 13 children had moved during the year. how many total students were left at the end of the year?

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?