Based on chargaff's rule, the percentage of cytosine in the dna of the bacterium s. lutea in figure 12-3 should be around what? the answer is 36.6%, but i don't know why. what's the reason why the answer is 36.6%?

You have to remember Chargaff's rule which is pretty simple:
A always pairs with T; G always pairs with C
So look at your table. 
Human: you know 2 numbers: G is 19.9 and T is 29.4. By Chargaff's rule you can fill in the other 2 so you'd have:
A (29.4), G (19.9), T (29.4), C (19.9)
I'll skip to the last one on the bacterium:
Here you only know one. But, you really know 2: 
A is 13.4 so T must also be 13.4. This adds up to 26.8.
100-26.8= 73.2 (which is the amount of G+C); so divide this by 2 and you get 36.6 - so 36.6 G and 36.6 C!
Just remember who pairs with who...will never let you down!

It may have some of the same features that it’s mother does but, it may have some of the same features that it’s father has as well.