I have the following order of questions:
1 (multiple choice)- Choose the animals in your herd:
a)cow; b)duck; c)goose

example: only two animals have been chosen

2 (integer)- How many cows do you have? 8
3 (integer) - How many ducks do you have? 8

4 - Total number of animals: 16

I would like to add up the values given in questions 2 and 3, if the answer is that there are cows and ducks on the farm, but if the person selects a goose, the question would add up the value. My questionnaire only adds up if I select all the multiple choice options.

Hi @barata_clicia1 I faced a similar challenge sometime this week.
In this case, it appears that the calculation expression is not being fulfilled since there are missing values in the overall expression.

My Solution
To solve this, I implemented the Default Field and equated the unselected options to zero to fulfil the expression.

It would then be necessary to create an IF logic code, so that if the other options are not selected, they are considered to have a value of zero, but how would I do this?