A drilling machine has to be designed with 8 spindle speeds

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A drilling machine has to be designed with 8 spindle speeds ranging approximately between 120 to 1200 rpm. The 5th spindle speed is

1. 445
2. 480
3. 620
4. 865

Correct Option: A

N_min = 120 N_max = 1200
N = 8

N₅ = N₁ r⁴ = 120 × 1.384⁴
N = 447rpm