Answer:
They didn't put a 0 for the one's digit after multiplying 335 and 20.
Explanation:
Your multiplication is right.
Let me write out what you did again:
3.35
* 2.9
--------
3015
+6700
------------
9715
So the answer is 9.715 .
Now what could the person have done to get the 3.685?
A common error that some students forget to add is a 0 for the product of 335 and 20.
That is, what do you get when you do:
3015
+670
-------
3685
Then moving the decimal over 3 times gives: 3.685.
So they forgot to add a 0 when moving over to multiply the top digits to the tens digit of the second number.