Answer:
The answer to your question is 6.0 moles of O₂
Step-by-step explanation:
Data
 2KClO₃ ⇒ 2KCl + 3O₂ 
moles of O₂ = ?
moles of KCl = 4 
Process
To find the number of moles of O₂, use proportions and cross multiplication. 
Use the coefficients of the balanced equation.
 2 moles of KCl ----------------- 3 moles of O₂
 4 moles of KCl ----------------- x
 x = (4 x 3) / 2
-Simplification
 x = 12/2
-Result
  x = 6 moles of O₂
-Conclusion
When 4,0 moles of KCl are produced, 6.0 moles of O₂ will be produced.