m4maths Maths Puzzle

Pls check the solution for puzzle

Total no. of words of 5 letters formed by letters of the word BOMBAY.
[You should use only 5 letters out of 6 letters of the word BOMBAY]

solution given is ..................
.....
(1st case all the letters are different
total no. of words of this category = 5*4*3*2*1 = 120
2nd case B comes twice and 3 others
total no. of words of this category = 5*4*3*2*1/2*1 = 60
so total words = 120+60 = 180)

I think answer should be 360
as
solution should be

1st case all the letters are different
total no. of words of this category = 5*4*3*2*1 = 120
2nd case B comes twice and 3 others
total no. of words of this category = 4*(5*4*3*2*1/2*1) = 60*4=240 because there will be 4 combinations of such words.
so total words = 120+240 = 360

pls excuse me if I am wrong.

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m4maths Other Question

Total no. of words of 5 letters formed by letters of the word BOMBAY.
[You should use only 5 letters out of 6 letters of the word BOMBAY]
There are 50 coins. Half of them are genuine and the other half are counterfeit. Genuine coins weigh 10 grams, while counterfeit coins differ from the genuine coins by 1 gram (some of them weigh 9 grams, and some of them weigh 11 grams). You have a balance scale with two pans. In each weighing the scale shows the difference of the weights placed in each pan. You will randomly choose a coin. What is the minimum number of weighings necessary to guarantee to determine whether it is genuine or not?