What units do i use for the question:
how far does a lorry move if it is travelling at 15m/s for 20 mins

You can use any distance unit you want.  But whatever unit you choose,
you'll need to EITHER change the 15 m/s and the 20 mins to be consistent
before you multiply them, OR change the answer to your chosen unit after
you multiply.

Examples:

#1).  I choose the unit of meters.

    (20 mins) x (60 sec/min)  = 1200 sec

    (15 m/s) x (1200 sec)  =  (15 x 1200) m-sec/sec  =  18,000 meters .

                                                                            =  18.0 kilometers

#2).  I choose the unit of feet.

       (15 mtr/s) x (3.28 ft/mtr)  =  49.2 ft/sec

                (49.2 ft/sec) x (20 min) x (60 sec/min)

             =   59,055 feet x (1 mile / 5,280 ft)

             =    11.2 miles