Please help, It's for my test.

Sherina wrote and solved the equation. x-56=230 x-56-56=230-56 x=174 what was sherina’s error? sherina’s work is correct. sherina only needed to subtract 56 from 230. sherina made a subtraction error when subtracting 56 from 230. sherina should have added 56 to both sides of the equation.

Anyone know? will mark brainliest if correct!

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?