* x denote the number of days we jogged in the morning and stayed at home in the evening;
* y denote the number of days we played tennis in the evening and did nothing in the morning; and
* z denote the number of days we neither jogged nor played tennis.
y + z = number of mornings we did nothing = 9
x + z = number of evenings we stayed at home = 14
x + y = number of days we jogged or played tennis = 13
Adding the above three equations and dividing both sides by 2 gives
x + y + z = 18
Since there are only three types of days, the total number of days I stayed with my cousin is their sum, i.e., 18.