There are 13 women who are 50 years old in the data set. To obtain the answer to question 4, you could have used one of the following commands:
list hhid if age == 50 & gender_n == 2
ORcount if age == 50 & gender_n == 2
The first command lists out the hhid values for all observations with a value of age equal to 50 and a gender_n value equal to 2 (which represents females). Thus using qualifiers we were able to list information for a specific group (in this case 50 year old women). Similar to one of the earlier exercises, the variable hhid could be replaced with any other variable in the data set, as we are not interested in the listed values themselves, but merely how many values are listed.
If you don't need specific information about the 50 year old women in the data set, but rather just need a count of the number of individuals who possess the characteristics mentioned, then the count if command is useful. This command is preferable if there are so many observations that it makes it difficult to count them through the list command.