Number of yellow bikes =ny Number of green bikes =ng We know that the total number of bikes is 100 : ny+ng=100 The average speed of all bikes is given by the weighted average of the speeds of yellow and green bikes: (ny×. Average speed of yellow bikes +ng× Average speed of green bikes) / Total number of bikes = Average speed of all bikes (ny×55+ng×30)∕100=35 Multiply both sides by 100 : 55×ny+30×ng=3500 Now we have a system of two equations: ny+ng=100 55×ny+30×ng=3500 From equation 1 , we can express ny as: ny=100−ng Substitute this into equation 2 : 55×(100−ng)+30×ng=3500 5500−55×ng+30×ng=3500 5500−25×ng=3500 Rearrange to solve for ng : 25×ng=5500−3500 25×ng=2000 ng=2000∕25 ng=80