The perimeter of a semi circle is 20.56 mm. what is the semicircle of radius?

The computer that controls a bank's automatic teller machine crashes a mean of 0.6 times per day. what is the probability that, in any seven-day week, the computer will crash less than 5 times? round your answer to four decimal places

What is the measure of angle a and angle c ?

Me because i don't understand how to do it.: )