A and B are walking on the edge of a circular playground, starting from the same place and walking back to back. A Walk 40 meters per minute. B didn't walk 38.5m in minutes. ...

40*4= 160

38.5*4= 154

154+ 160=3 14

This is the length of the playground.

2R*3. 14=3 14

R=50

50*2= 100

Extended data:

Formula related to circle:

1, circular area: S=πr? ，S=π(d/2)？ . (d is the diameter and r is the radius).

2. area of semicircle: s semicircle = (π r 2)/2. (r is the radius).

3. Ring area: s great circle -S small circle = π (r 2-r 2) (r is the radius of great circle and r is the radius of small circle).

4. Circumference: C=2πr or c = π d..(d is the diameter and r is the radius).

5. The circumference of a semicircle: d+(πd)/2 or d+π r (d is the diameter and r is the radius).

6. The area of the circle where the sector is located is divided by 360 and multiplied by the angle n of the central angle of the sector, as shown below:

S=n/360×πr？

S=πr？ ×L/2πr=Lr/2(L is arc length and r is sector radius)