While Christians fast, do they only focus on worshipping God (such as bible reading and praying)? If they involve in any other activity besides worshipping God, would fasting be inefficient? What did the people from the bible do on Fasting days?
The Church Fathers have referred to fasting without prayer as "the fast of the demons" since the demons do not eat according to their incorporeal nature, but neither do they pray.
From my understanding, the Orthodox are the only large denomination that participate in regular fasting. Almost every Wednesday and Friday include a vegan diet (except shrimp, scallops, etc.) with no alcohol or olive oil. The entirety of lent is like this as well, except alcohol/olive oil is permitted on Saturday and Sunday and only one meal is permitted per day; in addition, no food at all is permitted the first three days and the the final two days of Lent).
During these periods of fasting, they do not "only focus on worshiping God", as they more than likely work and have families to take care of. However, it is expected that they worship much more than usual, as fasting without worship is worthless (i.e., inefficient).
You can think of it as a continuum as opposed to a black or white answer. If one was to fast and worship 100% of the day, it would be more efficient than one who fasted and worshiped only 30 minutes of the day, which would be more efficient than one who did not fast and worshiped 5 minutes out of the day.
This answer is in regards to the Orthodox perspective, as the Catholics and Protestants don't engage in regular fasting. (The Catholics used to fast but relaxed it to the restriction of no red meat on Fridays, during Lent).
Jesus called us to fast in secret.
Matthew 6:17-18 ESV
But when you fast, anoint your head and wash your face, that your fasting may not be seen by others but by your Father who is in secret. And your Father who sees in secret will reward you.
I interpret this to mean that Christians who are fasting should continue their day without drawing undue attention to the fast they are undergoing.
If you are supposed to go somewhere, go. If you are supposed to do something, do it.