QuestionLena starts to use the standard algorithm to solve $39 \times 4$. Her work is shown below. $$\begin{array}{r} m \\ 39 \\ \times \quad 4 \\ \hline 6 \end{array}$$
What number should Lena replace $m$ with? $\square$

Studdy Solution
Multiply the leftmost digit of $39$ (which is $3$) by $4$ and add the carry-over $3$.
$$3 \times 4 + 3 = 12 + 3 = 15$$
Therefore, $m$ should be $15$.
The number Lena should replace $m$ with is:
$\boxed{15}$