If the refrigerators are denoted by GGGGBB: four good ones and two bad.
There are 15 ways to rearrange that string.
a) BGGBGG, GBGBGG, GGBBGG represent the only strings in which the last defective refrigerator is found on the fourth test. So what is the probability?
b) What strings represent “no more that four refrigerators need to be tested to locate both of the defective refrigerators”?
See if you can do part c.