Answer :
Td1= First half distance
Td2=Second half distance
Td1 = 3m/s
Td2(1) = 4.5m/s
Td2(2) = 7.5m/s
Td2 Average = (4.5+7.5)/2=6m/s
Total average =(3+6)/2= 4.5 m/s
Td2=Second half distance
Td1 = 3m/s
Td2(1) = 4.5m/s
Td2(2) = 7.5m/s
Td2 Average = (4.5+7.5)/2=6m/s
Total average =(3+6)/2= 4.5 m/s
<----------------------4--------------------->
A------------------B------------C-------------D
<----2------------> <-----1----> <-----1---->
LET TOTAL DISTANCE=4
FROM A-B
SPEED=3M/S
TIME=2/3
FROM B-C
SPEED=4.5M/S
TIME=1/4.5
FROM C-D
SPEED=7.5 M/S
TIME=1/7.5
AVERAGE SPEED=TOTAL DISTANCE COVERED /TOTAL TIME TAKEN
=4 / 2/3+1/4.5+1/7.5
=4/ 46/45
=3.9m/s