The strength of the cubic concrete of the first  production line is 120 N/mm² with standard deviation of 15 N/mm², and for the second production line  is 100 N/mm² with standard deviation of 10 N/mm². If two samples are selected from each line with n=30 for each and the strength was tested. Find the probability that the strength of first line is greater than the second line by:

a- 12 N/mm² or more

b- 25 N/mm² or more.