Question

on a 120 km track,a train travel the first 30 km at a uniform speed of 30 km/h . how fast must the train travel to cover next 90 km . when the average speed is 60 km/h for the entire trip

Answer 1

Speed of the train in the first 30 km, v1 = 30 km/h
Speed of the train in the next 90 km, v2 (to be determined)
Average speed = 60 km/h = (v1 + v2)/2
= (30 km/h + v2)/2

⇒ v2 = 90 km/h

Answer 2

ᴀɴsᴡᴇʀ                                                            

                            ɪɴ ᴛʜᴇ ғɪʀsᴛ ᴋᴍ sᴘᴇᴇᴅ ᴏғ ᴛʜᴇ ᴛʀᴀɪɴ, ᴠ1 = 30 ᴋᴍ/ʜ                            

                            sᴘᴇᴇᴅ ᴏғ ᴛʜᴇ ᴛʀᴀɪɴ ɪɴ ᴛʜᴇ ɴᴇxᴛ 90 ᴋᴍ, ᴠ2                            

                           ᴀᴠᴇʀᴀɢᴇ sᴘᴇᴇᴅ = ᴋᴍ/ʜ = (ᴠ + ᴠ)/2                                                                              

→ ᴠ2 = 90 ᴋᴍ/ʜ