The following questions are accompanied by three statements I, II and III. You have to determine which statement(s) is/are necessary/sufficient to answer the question. What is the relative speed of two trains running in opposite direction? (I) The length of each train is 120 m. (II) The speed of one of these trains is 48 km/hr. (III) They take 8 seconds to cross each other.

When trains cross in opposite directions, relative speed = (sum of lengths)/time. Statement III gives time (8 seconds) which is essential. Statement I gives total length (120 + 120 = 240 m) which is essential. Statement II gives one train's speed (48 km/hr) but we need the sum of both speeds. However, if we interpret it as needing all statements to have enough data, we need length and time and at least one speed to find the other. The question asks for relative speed, and with only II, we have insufficient data (missing lengths and time). All three together provide complete information.