During the turning of a $20$ mm- diameter steel bar at a spindle speed of $400$ rpm, a tool life of $20$ minute is obtained. When the same bar is turned at $200$ rpm, the tool life becomes $60$ minute. Assume that Taylor's tool life equation is valid. When the bar is turned at $300$ rpm, the tool life (in minute) is approximately.

1. $25$
2. $32$
3. $40$
4. $50$