15.700

1.57

15 is more than ten times a tenth so the first digit is the whole number 1
then there are still 5 tenths to place after the decimal in tenths
the next column is the hundreths column
conveniently the 7 has the the column to itself

if it were 17 hundreths the answer be 5.67

15.700

15/10 = fifteen tenths or 1.5 (divide 15 by 10)
7/100 = seven one hundreths. dealing with hundreths is like saying 7 cents since there are 100 cents in a dollar) so it is written 0.07, but you can still divide 10 into 7 and that will give you your decimal point because 100 is a bigger number than 7.
hope this helps you understand. keep practicing.