h -> min : 60 min/h : the conversion factor for changing hour to min.

min -> s : 60 s/min : converts minute to seconds

and here comes the conversion of 24 h to seconds :

24 (h) * 60 (min/ h)* 60 (s/min) = 86400 s !

You can treat units like numbers for simplification thus the hour goes with hour and minute goes w/ minute and only remains second as the unit.

To convert 25 kg to mg you can convert in two steps, kg to g and then g to mg, here are conversion factors :

kg -> g : 1000 g/kg

g -> mg: 1000 mg/g

25 (kg) * 1000 (g/kg) * 1000(mg/g)= 25000000 mg

Again you treat units like real numbers for simplification.

G.L!

25kg x 1,000g/kg x 1,000mg/g = 25,000,000mg (kg & g cancel)