this all has to do with place value...
0.37....the last number (7) is in the hundredths place....so remove the decimal and put it over 100......the fraction would be 37/100...and the denominator is 100
and IF it would have been 0.037....the 7 would be in the thousandths place and u would put it over 1000.....so it would be 37/1000.
Understand ?
