Question

fill in the missing number:
2 1
× 4 1
------
□ □
+ 8 4 0
------
8 6 1

Explanation:

Step1: Find the missing two-digit number

Let the missing number be $x$. Use the final sum to solve for $x$:
$x + 840 = 861$
$x = 861 - 840$

Step2: Calculate the value of $x$

$x = 21$
(Verification: $21 \times 1 = 21$, which matches the first partial product of the multiplication $21 \times 41$)

Answer:

21