A semicircle is half of a circle =3. 14* radius r = 3.14 *10 = 31.4.

2.40cm = 0.4m.

Circle 2*3. 14*0.4=2.5 12 (circumference)

You can walk100 * 2.512 = 251.2 per minute (100 times the circumference).

The total length is 25 12m, so the required time is 2512/251.2 =10 minute.

The general formula is

25 12÷(0.4×2×3. 14× 100)= 10

3。 The diameter is 80m, so the circumference = 80 * 3.14 = 251.2m..

They go in opposite directions, so the speed is 31.8+31= 62.8 m/min.

The meeting time is 25 1.2/62.8=4 minutes.

The general formula is

80×2×3. 14÷(3 1.8+3 1)=4