How can I solve these two problems?(adsbygoogle = window.adsbygoogle || []).push({});

P(n,3)=210 and P(5,r)=20

For the first one I got up to n(n-1)(n-2)=210 but I dont know how to solve a cubic equation...And the second one I have no clue. I'd appreciate some help, thanks

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Permutations of n taken r at a time

**Physics Forums | Science Articles, Homework Help, Discussion**