Mr. Nguyen had 5/8 of a gallon of gasoline. He used 1/2 of a gallon in his lawn mower. How much gasoline does Mr. Nguyen has left?

A.
4/8 of a gallon
B.
1/8 of a gallon
C.
1/6 of a gallon
D.
4/6 of a gallon

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?