I have two questions I'm stuck on.

An object made entirely from one material has a volume of 30m(3). It floats in a liquid of density 3g/cm(3) with only half its mass below the liquid's surface. The object is then placed in another liquid of the same volume but with a density of 1g/cm(3). If the final upthrust that will act on the object is expressed in the form a x 10(b)N give the values of a and b. (Assume g = 10 m/s(-2).)

The answer is a=b=3. I've no idea how to do it.

Also I have another question.

A study has claimed to show that abnormal blood levels of prostate specific antigen (PSA) may be an indicator of prostate cancer. Twenty percent of men with prostate cancer have normal PSA levels and at least 2 out of 3 men with abnormal PSA levels do not have prostate cancer. Tests were performed on a group of 2500 men and 1051 of them had normal blood PSA levels. What is the maximum possible number of men you would expect to have prostate cancer?

A- 200

B- 241

C- 483

D- 604

E- 724

Could anyone answer these questions for me and show the working. A huge thank you in advance.