Multiply 156 by 4=4×100+4×50+4×6=400+200+24=624. This is the same as 624.0.
Now count the number of digits after the decimal points: 2+1=3.
Move the decimal point 3 places to the left of where it is in 624.0 and we get .624 which we write as 0.624 so as not to lose the decimal point when reading.