Spot speed studies were conducted on a certain stretch of a highway and consolidated data collected are given below:

Speed range, Kmph	No. of vehicles observed
0 - 10	10
10 - 20	40
20 - 30	50
30 - 40	50
40 - 50	150
50 - 60	250
60 - 70	300
70 - 80	90
80 - 90	40
90-100	20

Determine the upper and lower values of speed limits for the regulation of mixed traffic flow.