A really hard question (for me at least) of my homework

Question

The research and development department of a manufacturing company has identified 4 possible types of defects for a new product (A, B, C, D). The identified probabilities are 0.4, 0.3, 0.05, and 0.25, respectively. Of these defects, type D is the most serious and makes it impossible to use the product. The other defects have a solution. A pilot production program of 50 units will be launched and the maximum quality limit has been established as the mathematical expectation of defect D plus one standard deviation.

Answer 1

how about this...

then 12.5 of them are expected to break.

stnd dev = sqrt ( not damaged or repairable * 0.75 + lost product * 0.25)

sqrt(37.5*0.75 + 12.5*0.25) = 5.5901699437494742410229341718282....

12.5+ 5.59+ = 12.5 + 5.6 = 18.1 ---> 19 units lost shall be tolerated

50-19 = 31 must pass