Dear all, I have three problems that need all of you help to fix

I could not sum value for skipping question
e.g. I have 3 questions ask about income and those 3 questions have
parent questions.
If Q1 answer 'yes' we will ask Q1.1 about how much money?
If Q2 answer 'yes' we will ask Q2.1 about how much money?
If Q3 answer 'yes' we will ask Q3.1 about how much money?
Else, if 'no' it will skip to next question
Then I will calculate Total= ${Q1.1} + ${Q2.1} + ${Q3.1}
But in case among which one, both, or all question has skipped, it
could not calculate the total. Even though I set the default value to those
variable. It still could not calculate 
I want to set default for select one question based on conditions
e.g. If Total of income >=300, ${fieldname}='yes'
else ${fieldname}='no'
I used this formula
*if(${total_income}>=300, selected${select_income},'1') *This
formula could not meet with else condition. So how can I do? 
I want to use relevant for group based on three conditions
e.g. If meet with this condition below, it will show group. And else,
it will hide. I used this formula. That is correct?
selected(${fieldname},'0') and selected(${fieldname},'0') and
selected(${fieldname},'0')
Thanks in advance for your responses
Best,
Lihol