suppose a certain combination lock requires three selections of numbers, each from 1 through 20. (a) how many choices are there for the first number in the combination? how many choices are there for the second number in the combination? how many choices are there for the third number in the combination? how many different combinations are possible? (b) suppose the locks are constructed in such a way that no number may be used twice. how many choices are there for the first number in the combination? how many choices are there for the second number in the combination? how many choices are there for the third number in the combination? how many different combinations are possible?