Answer:
There is a 62.7% probability that a transaction can be completed in less than 100 milliseconds.
Explanation:
This a probability problem.
We have the following probabilities:
-70% probability that a transaction is an addition of an item.
-30% probability that a transaction is a change to an item
-81% probability that an addition can be completed in less than 100 milliseconds
-20% probability that change can be completed in less than 100 milliseconds
The probability P that a transaction can be completed in less than 100 milliseconds is:

In which
is the probability that the transaction is an addition and it takes less than 100 milliseconds. So

is the probability that the transaction is a change and it takes less than 100 milliseconds. So

So

There is a 62.7% probability that a transaction can be completed in less than 100 milliseconds.