The most important thing to remember
when adding decimals is to
write the problem vertically,
lining up the corresponding place values,
and that's also true when
we're adding a whole number
and a decimal.
We have 15 plus 5.124.
Notice how this five
and this five are both
in the ones place value.
So to write this vertically, we'll have 15
plus 5.124.
And now we can also recognize
that the decimal point
for 15 would be to the
right of this five here.
And now if we want we can
add zeroes to the right
of the decimal point without
changing the value of 15.
Now that we have this
sum written vertically
and all the corresponding
place values are lined up,
we want to bring the decimal
point down into our sum
and then add as we normally would.
So we have four, two, one.
This would be 10, carry a one,
and we have a two in the tens place value.
So the sum is 20.124,
or 20 and 124 thousandths.
