Let O represent the set of civil servants working in the office. Similarly, let F represent the set of civil servants working in the field.

According to the question, 65 civil servants work in the office, i.e., $\rm n(O) = 65$, and 50 work in the field, i.e., $\rm n(F) = 50$.

Also, there were a total of 90 civil servants, $\rm n(U) = 90$. 35 of them worked in both office and field, $\rm n(O \cap F) = 35$. A few civil servants may be absent, $\rm n (\overline{O \cup F})$.

1) The required values of $\rm n(F)$ and $\rm n(O \cap F)$ are $\rm n(F) = 50$ and $\rm n(O \cap F) = 35$.

2) Representing the above information in a Venn diagram.

3) Let $\rm n (\overline{O \cup F})$ represent the set of civil servants who were absent. Using the formula

$\rm n(U) = n(O) + n(F) - n(O \cap F) + n (\overline{O \cup F})$

$\rm or, n (\overline{O \cup F}) = n(U) - n(O) - n(F) + n(O \cap F)$

$\rm or, n (\overline{O \cup F}) = 90 - 65 - 50 + 35$

$\rm \therefore n (\overline{O \cup F}) = 10$

Hence, 10 civil servants were absent.

4) We can obtain the number of civil servants working in fields only by using the formula $\rm n_o{F} = n(F) - n(O \cap F)$

$\rm \therefore n_o{F} = 50 - 35 = 15$