A probability question on time 2

A and B decide to meet up at a place.Each of them shows up at a random time b/w 4 and 5 pm.They decide that they will wait exactly 20 mins for the other person whosoever comes first until 5 pm.The watch that each of them is wearing is only able to display the time in minutes rounded off to the nearest integer . Which means that if in the real timeline the time gone by is 20.44545 mins the watch will show 20 mins. And if the time gone by is 20.66677 mins then the watch will show as 21 mins. For clarification if the time gone by is 20.5 mins then the watch will show 21 mins. If the probability that the meet is $\frac{a }{b}$ where a and b are coprime positive integers. Then find the value of $a+b$ .