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.