Quantity A: When order doesn't matter, we have to select 3 out of 6 people and can arrange those 3. Selecting 3 out of 6 people is 6C3 and then arranging three people in a row would be 3!.
So, A = 6C3 * 3! = 6*5*4 = 120

Another way of doing this is that suppose we have three positions to be filled. The first position can be filled in 6 ways, second can be filled in 5 ways and third in 4 ways. And since these have to happen together, we would take the product of 6, 5 and 4. So Quantity A would be 6*5*4 =120

Quantity 2: Simply selecting 3 people out of 10 i.e. 10C3 = 120.

Hence Quantity A equals B

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