A few basic probability question

Q: A large group of people is to be checked for two common symptoms of a certain disease. It is thought that 20% of the people possess symptom A alone, 30% possess symptom B alone, 10% possess both symptoms, and the remainder have neither symptom. For one person chosen at random from this group, find these probabilitys:

a) The person has neighter symptom.

b) The person has at least one symptom.

c) The person has both symptoms, given that he has symptom B.

A: I am assuming these events are independent.

a)

b)

c)

The book says teh answers are:

a).4

b).6

c).25

I am not sure what I am doing wrong.

Thanks