Let x be the number of ppl exceeding the speed limit
motorists who exceed the posted speed limit do not receive speeding tickets =0.2x
motorists exceed the posted speed limit and receive speeding tickets = 0.1
Therefore, total exceed = Those who exceed and don't recieve ticket + Those who exceed and recieve ticket.
It will be easier to visualize if you draw a 2x2 grid with recived tickets/not recieved tickets Vs exceeded/not exceeded.
One needs to be careful dealing with these problems containing statements like these:
1) "but 20% of the motorists who exceed the posted speed limit do not receive speeding tickets" - gives 0.1+0.2x=x => 12.5%
2) "but 20% of the motorists exceed the posted speed limit and they do not receive speeding tickets" - gives 0.1+0.2=0.3 =>30%
To find what you seek in the road of life, the best proverb of all is that which says:
"Leave no stone unturned."
-Edward Bulwer Lytton