Question

The initial concentration of a reactant in a first order reaction is 0.620 molar. What will be its concentration after 3 half-lives?

I did 0.620 -> 0.31 ->0.155

0.155M will be the concentration. Is this correct?

Answer 1

You're ok as far as you went but you only went through 2 half lives.
0.620 start becomes 0.310 after 1.
0.310 becomes 0.155 after 2.
0.155 becomes ..... after 3.

Answer 2

Oh ok I did realize that. Now I got 0.0775M