The probability of a train arriving on time and leaving on time is 0.8. the probability of the same train arriving on time is 0.84. the probability of this train leaving on time is 0.86. given the train arrived on time, what is the probability it will leave on time?

No answer is possible, because the given information is inconsistent.

If the arrival probability is 0.84 and the departure probability is 0.86,
then the probability of the same train arriving AND leaving on time
is

         (0.84) * (0.86)  =  0.7224       
 
                             NOT  0.8 , like the first sentence says.

You can find the area of a rectangle by multiplying the length and height. so multiply 1/2*7/4(which is the simpler version of 1 3/4) and you get 7/8 as your answer.