According to the information given in the exercise, you know that he cycled a total of of 16 kilometers by making 4 trips to work.
Let be "d" the total amount of kilometers Mr. Santos will have cycled after 5 trips to work.
Based on the above, you can set up the following proportion:

Finally, you must solve for the variable "d" in order to find its value. This is:

Therefore, the answer is:
![20\operatorname{km}]()