In a sequence of numbers, each number is 7 less than 3 times the previous number. For example, if the first number is five, the second number would be 8 (3 x 5 - 7) and the third number would be 17 (3 x 8 - 7). If the third number in the sequence was 53, what was the first number in the sequence?

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?