MathematicsHard98×since 2002Q4838Let 0≤r≤n0 \leq r \leq n0≤r≤n. If n+1Cr+1:nCr:n−1Cr−1=55:35:21{ }^{n+1} C_{r+1}:{ }^n C_r:{ }^{n-1} C_{r-1}=55: 35: 21n+1Cr+1:nCr:n−1Cr−1=55:35:21, then 2n+5r2 n+5 r2n+5r is equal to :A62B60C55D50Check answerSkip