Determine the RSA private key from public key I was told that you can determine the private key of an RSA encryption with the public key.

I thought the point of a public key was to avoid letting people find the private key, but anyway, they said I could find it by simply factoring the public key modulus? Were they joshing me or can it be done? If so, how would I do that (need to acquire $p,q$ somehow?)? 

So with with example Public Key $( e, n ) = ( 13, 119 )$ or any other valid key you want to use...
I was told that you can determine the private key of an RSA encryption with the public key. Were they joshing me or can it be done?


Yes, it can be done. What you have not been told is that to factor a public key (usually hundreds of digits) to find the private key,  requires a time exponential in the length of the public key, 
therefore even a supercomputer could take years, if not centuries. Factorization is believed to be a Hard Problem.
 On the other hand, if you already know the private key, you can get the plaintext in very few CPU's cicles. 
This is the general principle underlying asymmetric cryptography: