2.10 *1/2÷ (15-10) =1hour

1*15 =15km

3. (100+80) * 5 = 900m

900/( 100-80)=45 minutes

Plus five minutes back, that's 50 minutes.

That is 8: 20.

4 4. 10/0km =10000m

Suppose A starts to slow down after X minutes, and then walks for Y minutes.

So B walked for X+Y-45-5=X+Y-50 minutes.

Then there is

104X+84Y= 10000

2 10(X+Y-50)= 10000

that is

26X+2 1Y=2500

2 1X+2 1Y- 1050 = 1000

that is

26X+2 1Y=2500

2 1X+2 1Y=2050

solve

x = 90°

That is, after 90 minutes of departure, we will start to slow down.