Pls further explanation o bool n double

Question

book is online =true

Answer 1

variable name[here isOnline is variable_name] can't contain spaces
bool isOnline=true;
//for C# language
https://www.sololearn.com/learn/CSharp/2582/?ref=app

Answer 2

Double:
https://www.sololearn.com/learn/CPlusPlus/1622/?ref=app
Bool:
https://www.sololearn.com/learn/CPlusPlus/1623/?ref=app