Question 1173585
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Hi,
13 Items:  6 ShopA   4 ShopB     3 ShopC
Choosing 3, find the number of combinations in each of the following cases.
nCr = (n!)/(r!(n - r)!).   nCr on Your Caclulator
A. Choosing 3   items from the same shop: 6C3 + 4C3 + 3C3 =   20+4+1 = 25
B. Choosing 3 items which are not from shop A:  7C3 = 35
C. Choosing at least 1 item from shop A:  
  (6C1)(7C2) + (6C2)(7C1) + (6C3) = (6)(21) + (15)(7) + (20)
Wish You the Best in your Studies.