Given,
A be the event that the absolute difference between two randomly chosen real numbers in the sample space [0,60] is less than or equal to a,
So, ∣x−y∣<a⇒−a<x−y<a
⇒x−y<a and x−y>−a
Now on plotting the diagram we get,

Now from diagram probability is given by,
P(A)=area(OBDF)area(OACDEG)
⇒P(A)=area(OBDF)area(OBDF)−area(ABC)−area(EFG)
⇒3611=3600(60)2−21(60−a)2−21(60−a)2
⇒1100=3600−(60−a)2
⇒(60−a)2=2500⇒60−a=50
⇒a=10