help me for this sum?

It seems to be a bit clumsy but the question itself is just a bit of equation. The key to the solution is "how to organize the info in a SYSTEMATIC way & transcribe a lump of words into mathematical expressions". That's what I'm trying to demonstrate.

[Variables] (minimizing no. of variables)
Let N be the total no. of days of stay (i.e. the target answer);
and J be the no. of mornings spent on jogging,
(11-J) be the no. of evenings spent on tennis.

[Illustration] (listing out known situations & relationships: the MOST important part yet often badly done)
The possible situations of spending 1 day are:
. . . . . . . . . .(i) . . . .(ii) . . . . .(iii)
1. morning . idle . jogging(J) . idle
2. evening . .idle . . idle . . . .tennis(11-J)

Other known info:
No. of idle ...
- 3. mornings: 12
- 4. evenings : 17
No. of mornings = N
No. of evenings = N (quite silly though...)

[Equations] (finally... constructing equations using the established info)
Based on 1 & 3, we have: N - J = 12
Based on 2 & 4, we have: N - (11-J) = 17

Solving the simultaneous equations,
N = 20; J = 8

Therefore, the stay is 20 days.

* * *
Hope you don't find it boring reading such a long answer.
Please notify me if you also want the steps for solving the equations above.

20 days

20 days

3 evening for tennis
8 morning for jogging