Speed × time = distance Calculation: In the 1st 20min the thief cover distance =4m, 20 min in hour =20∕60 hour Let's assume that the speed of security guard =xm∕hr, where x>12 According to the question, ⇒(x−12)×20∕60=4 ⇒x−12=12 ⇒x=24 ∴ The correct answer is 24m∕h