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Luis is packing a bag for vacation. He has 9 unique shirts, but he can only fit 5 in his bag.
How many different groups of 5 shirts can he take?

Luis is packing a bag for vacation. He has 99 unique shirts, but he can only fit 55 in his bag.\newlineHow many different groups of 55 shirts can he take?

Full solution

Q. Luis is packing a bag for vacation. He has 99 unique shirts, but he can only fit 55 in his bag.\newlineHow many different groups of 55 shirts can he take?
  1. Identify Problem Type: Identify the type of problem.\newlineWe need to determine the number of combinations of 55 shirts that can be chosen from a set of 99 unique shirts. This is a combinatorics problem, specifically a combination problem where order does not matter.
  2. Use Combination Formula: Use the combination formula to calculate the number of different groups.\newlineThe formula for combinations is C(n,k)=n!k!(nk)!C(n, k) = \frac{n!}{k!(n - k)!}, where nn is the total number of items, kk is the number of items to choose, and “!!” denotes factorial.\newlineFor this problem, n=9n = 9 (total unique shirts) and k=5k = 5 (shirts to fit in the bag).
  3. Calculate Factorial of nn: Calculate the factorial of nn, which is 9!9!.9!=9×8×7×6×5×4×3×2×1=362,8809! = 9 \times 8 \times 7 \times 6 \times 5 \times 4 \times 3 \times 2 \times 1 = 362,880
  4. Calculate Factorial of kk: Calculate the factorial of kk, which is 5!5!.5!=5×4×3×2×1=1205! = 5 \times 4 \times 3 \times 2 \times 1 = 120
  5. Calculate Factorial of (nk)(n - k): Calculate the factorial of (nk)(n - k), which is (95)!(9 - 5)!.(95)!=4!=4×3×2×1=24(9 - 5)! = 4! = 4 \times 3 \times 2 \times 1 = 24
  6. Substitute into Formula: Substitute the factorials into the combination formula to find the number of combinations. C(9,5)=9!5!(95)!=362,880120×24C(9, 5) = \frac{9!}{5!(9 - 5)!} = \frac{362,880}{120 \times 24}
  7. Perform Calculations: Perform the calculations.\newlineC(9,5)=362,880120×24=362,8802,880=126C(9, 5) = \frac{362,880}{120 \times 24} = \frac{362,880}{2,880} = 126
  8. Verify Calculations: Verify the calculations to ensure there are no math errors.\newlineRechecking the calculations: 362,880/2,880=126362,880 / 2,880 = 126. There are no math errors in the calculations.

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