this type of problems the formulae is (5*x + or -15)*(12/11) Here x is replaced by the first interval of given time here i.e 4 Case 1 : (5*x + 15)*(12/11) (5*4 +15)*(12/11) (20+15)*(12/11) 35*12/11=420/11=38 2/11 min. Therefore they are right angles at 38 2/11 min .past4 Case 2 : (5*x-15)*(12/11) (5*4-15)*(12/11) (20-15)*(12/11) 5*12/11=60/11 min=5 5/11min Therefore they are right angles at 5 5/11 min.past4.