After #7.6# days, radon-#222# has elapsed #7.6/3.8=2# half-lives. So, its original activity would be #2^2=4# times the amount of activity it has right now.
If it has #10# becquerels of activity right now, that must mean that its original activity was #10 \ "Bq"*4=40 \ "Bq"#.