Please someone verify my answers are right; not so sure about this one.

Of 50 employees of a store located in downtown Boston, 18 people take the subway to work, 12 take the bus, and 7 take both the subway and the bus. How may employees (a) take the subway or the bus to work? (b) take only the bus to work? (c) take either the bus or the subway to work? (d) get to work by some other means?

(a) 50-12+7=45
(b) 50-18+7=39
(c) 50-7=43
(d) 50-18-12-7=13

Someone helped answer it, but the answers are so different. I need someone to please explain if i'm wrong.

Question

Please someone verify my answers are right; not so sure about this one.

Of 50 employees of a store located in downtown Boston, 18 people take the subway to work, 12 take the bus, and 7 take both the subway and the bus. How may employees (a) take the subway or the bus to work? (b) take only the bus to work? (c) take either the bus or the subway to work? (d) get to work by some other means?

(a)  50-12+7=45
(b) 50-18+7=39
(c) 50-7=43
(d) 50-18-12-7=13

Someone helped answer it, but the answers are so different. I need someone to please explain if i'm wrong.