✪ Choose the correct option.
In a typical bike rally, every bike has to reach a speed of 20 m/min within a minute and it increase its speed by 20 m/min and the sum was 1600 m/min. Four minutes later, a new biker started running in the race. After another two minutes, the biker with the speed of 400 m/min stopped due to injuries and again two new bikers started running in the race. What is the current average speed?