Question 294671
Which equation did you solve to get a+b=60?
What is the ratio of their speeds at what time?
B keeps changing.
We aren't told anything about A's speed.
In 3 hours they have gone 210 km
but B goes 10 kph more each hour.
1st hour distance a+b
2nd hour a+b+10
3rd hour a+b+20
So we have 3a+3b+30=210
divide by 3
a+b+10=70
a+b=60
a=60-b
or 
b=60-a
But again when are we taking this ratio? 
It is different at each hour.