15.4=5n-4,
15.4+4=5n,
19.4=5n, same as 5n=19.4,
n=19.4/5=19.40/5=3.88 so n lies between integers 3 and 4.
Or, you could say that, because 19.4 lies between 15 and 20 (nearest multiples of 5), n must lie between 15/5=3 and 20/5=4, that is, between 3 and 4.