There are 2 mistakes in the working below.
(a) circle the 2 mistakes in the working.
[13 -4 -3-2)]+5 +10[5+(-3)]
= [9 - 6]+5+10(-2)
= 3+5+(-20)
(b) rewrite the correct working and hence, evaluate the answer.

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?