Targeting specific nth child

Hi everyone,

I figured that we could change specific elements within a class by using their nth child number (I'm not sure this sentence is correct I'm not still learning css). 

How do you go about finding the nth child number of an element? I tried looking everywhere using the inspect tool but didn't manage to figure this out.

Thank you in advance!

<div class="tuan">
  <div class="b1"></div>
  <div class="b3"></div>
  <div class="b3"></div>
  <div class="b4"></div>
</div>

div b1 will have: .tuan div:nth-child(1)

div b2 will have .tuan div:nth-child(2)

div b3 will have .tuan div:nth-child(3)

...