01given
A basket contains 7 distinct red apples (R), 5 distinct white apples (W), and 8 distinct oranges (O). Total fruits to select =5, with constraints: number of oranges nO≥2, red apples nR≥1, and white apples nW≥1.
02goal
Find the total number of ways to choose 5 fruits such that nR+nW+nO=5 under the conditions nR≥1, nW≥1, and nO≥2.
03approach
Since nR+nW+nO=5 with nR≥1, nW≥1, nO≥2, the minimum total is 1+1+2=4. There is only 1 fruit left to distribute. This yields exactly three mutually exclusive cases for the triplet (nR,nW,nO):
Case 1: (1,1,3)
Case 2: (2,1,2)
Case 3: (1,2,2).
04execute
Compute each case using combinations from distinct fruits:
Case 1: (17)×(15)×(38)=7×5×56=1960
Case 2: (27)×(15)×(28)=21×5×28=2940
Case 3: (17)×(25)×(28)=7×10×28=1960
Total ways =1960+2940+1960=6860.
✓verify
Check symmetry between Case 1 and Case 3, or group terms: (28)[(27)(15)+(17)(25)]+(38)(17)(15)=28×[105+70]+56×35=28×175+1960=4900+1960=6860.