The time interval when they will meet next at the starting point is the LCM (20, 30 and 35 ). Thus, the prime factorisation of 20,30 and 35 is 20=2×2×5=(2)2×(5)1, 30=2×3×5=(2)1×(3)1×(5)1 and 35=5×7=(5)1×(7)1 LCM (20,30,35) =(2)2×(3)1×(5)1×(7)1 =12×35 =420min =
420
60
h=7h Hence, all three runners will next meet after 7h at the starting point.