Answer:
29,140
Explanation:
We don't know what strategy was used for 2-digit numbers, so we'll answer this in sort of a generic way.
This is posted in the College math section, so we assume a calculator can be used.
There are numerous ways that multidigit multiplication "by hand" is taught. All of them have in common that every digit of one number must be multiplied by every digit of the other number, and the results added according to their appropriate place value. In some cases, written formats are used that help keep track of place value (see attached).
_____
In other cases, the practitioner is expected to keep track mentally. One can do sum-of-product math in one's head, adding results according to place value. Here is sort of a written version of what one might do mentally:
... 2·1·10000 +(2·2+3·1)·1000 +(2·4+3·2+5·1)·100 +(3·4+5·2)·10 +5·4
... = 20,000 +7,000 +1,900 +220 +20
... = 27,000 +1,900 +220 +20
... = 28,900 +220 +20
... = 29,120 +20
... = 29,140
In this particular product, the thousands-place digit needs to be revised twice and the hundreds-place digit needs to be revised once. One must actually remember two or three sums simultaneously to execute this process mentally.
