Hello, dear all!
I won't take too much of your time with this introduction.
From the attached table:
How many values ''munte=mountain'' are in the table under the column ''altitudine=altutude''?
What is the medium density of Daphnia based on the ''campie=plain (Daphnia collected from plain rivers) '' criterium?
What is the maximum value of the pH from the column ''nutrienti=Nutrients'' specifically ''mezotrof= Mesotrophic lakes''?
Thank you in advance,
Raul
This looks like a homework assignment. Please take a look at the homework policy (FAQ: Homework Policy) and the reproducible examples (FAQ: How to do a minimal reproducible example ( reprex ) for beginners) so that people can help you out better.
Not really homework. 
More like a food for thought exercise to get familiar with RStudio.
See the FAQ: How to do a minimal reproducible example reprex for beginners for when looking for more specific answers.
Using the built-in mtcars data
sum(mtcars$carb == 3)
#> [1] 3
median(mtcars[which(mtcars$carb == 3),"mpg"])
#> [1] 16.4
The second example is an exemplary pattern for the second and third questions.
Thank you! The purpose of this was to put me on the right path and not to solve it for me. That would have been totally useless. I wouldn't have learned anything.
That's why I have not classified it as homework. Homework is about a result.
I am here for the way there; building logical mechanisms to learn this program.
All the best everyone!
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