$55-6=4955-6=49$

$\begin{array}{c}55\\ \underset{‾}{-{}_{1}6}\\ 49\end{array}\backslash def\backslash arraystretch\{1.25\}\; \backslash begin\{array\}\{l\}\backslash ;\backslash ;\backslash ;\backslash ;5\backslash ;5\backslash \backslash \backslash underline\{-~~\_1\backslash ;6\}\backslash \backslash \backslash ;\backslash ;\backslash ;\backslash ;4\backslash ;9\backslash end\{array\}$

1. Subtract the ones: Since $55$ is less than $66$, you calculate $15-6=915\; -\; 6\; =\; 9$ instead and "memorize" the $11$ at the tens digit.

2. Subtract the tens: $5-1=45\; -\; 1\; =\; 4$

$8-4=48-4=4$

$19-2=1719-2=17$

$\begin{array}{c}19\\ \underset{‾}{-2}\\ 17\end{array}\backslash def\backslash arraystretch\{1.25\}\; \backslash begin\{array\}\{l\}\backslash ;\backslash ;\backslash ;\backslash ;1\backslash ;9\backslash \backslash \backslash underline\{-\backslash ;\backslash ;\backslash ;\backslash ;2\}\backslash \backslash \backslash ;\backslash ;\backslash ;\backslash ;1\backslash ;7\backslash end\{array\}$

1. Subtract the ones: $9-2=79\; -\; 2\; =7$

2. Subtract the tens: $1-0=11\; -\; 0\; =\; 1$

$33-22=1133-22=11$

$\begin{array}{c}33\\ \underset{‾}{-22}\\ 11\end{array}\backslash def\backslash arraystretch\{1.25\}\; \backslash begin\{array\}\{l\}\backslash ;\backslash ;\backslash ;\backslash ;3\backslash ;3\backslash \backslash \backslash underline\{-\backslash ;2\backslash ;2\}\backslash \backslash \backslash ;\backslash ;\backslash ;\backslash ;1\backslash ;1\backslash end\{array\}$

1. Subtract the ones: $3-2=13\; -\; 2\; =\; 1$

2. Subtract the tens: $3-2=13\; -\; 2\; =\; 1$

$10-0=1010-0=10$

$43-23-15=543-23-15=5$

Subtract from left to right:

$=(43-23)-15==(43-23)-15=$

$\begin{array}{c}43\\ \underset{‾}{-23}\\ 20\end{array}\backslash def\backslash arraystretch\{1.25\}\; \backslash begin\{array\}\{l\}\backslash ;\backslash ;\backslash ;\backslash ;4\backslash ;3\backslash ;\backslash \backslash \backslash underline\{-\backslash ;2\backslash ;3\}\backslash \backslash \backslash ;\backslash ;\backslash ;\backslash ;2\backslash ;0\backslash end\{array\}$

1. Subtract the ones: $3-3=03-3=0$

2. Subtract the tens: $4-2=24-2=2$

$=20-15=20-15$

$=5=5$

Alternative Solution

$43-23-15=543-23-15=\backslash ;5$

Subtract in one step:

$\begin{array}{c}43\\ -23\\ \underset{‾}{-1_{1}5}\\ 5\end{array}\backslash def\backslash arraystretch\{1.25\}\; \backslash begin\{array\}\{l\}\backslash ;\backslash ;\backslash ;\backslash ;4\backslash ;\backslash ;3\backslash \backslash -\backslash ;2\backslash ;\backslash ;3\backslash ;\backslash \backslash \backslash underline\{-\backslash ;1\_1\backslash ;5\}\backslash \backslash \backslash ;\backslash ;\backslash ;\backslash ;\backslash ;\backslash ;\backslash ;\backslash ;5\backslash end\{array\}$

1. Subtract the ones: Since $33$ is less than $(3+5)(3+5)$, calculate instead $13-(3+5)=13-8=513-(3+5)=13-8=5$ and "memorize" the $11$ at the tens digit.

2. Subtract the tens: $4-(2+1+1)=4-4=04-(2+1+1)=4-4=0$

$21-4-7-10=21-4-7-10=$

Subtract from left to right:

$=(21-4)-7-10=(21-4)-7-10$

Intermediate calculation: $21-4=1721-4=17$

     $\begin{array}{c}21\\ \underset{‾}{-{}_{1}4}\\ 17\end{array}\backslash def\backslash arraystretch\{1.25\}\; \backslash begin\{array\}\{l\}\backslash ;\backslash ;\backslash ;\backslash ;2\backslash ;1\backslash ;\backslash \backslash \backslash underline\{-~~\_1\backslash ;4\}\backslash \backslash \backslash ;\backslash ;\backslash ;\backslash ;1\backslash ;7\backslash end\{array\}$

1. Subtract the ones:  Since $11$ is less than $44$, calculate instead $11-4=711-4=7$ and "memorize" the $11$ at the tens digit.

2. Subtract the tens: $2-1=12-1=1$

$=(17-7)-10=(17-7)-10$

$=10-10=10-10$

$=0=0$

Alternative Solution

$21-4-7-10=21-4-7-10=$

Subtract in one step:

$\begin{array}{c}21\\ -4\\ -7\\ \underset{‾}{-1_{1}0}\\ 0\end{array}\backslash def\backslash arraystretch\{1.25\}\; \backslash begin\{array\}\{l\}\backslash ;\backslash ;\backslash ;\backslash ;2\backslash ;\backslash ;1\backslash \backslash -\backslash ;\backslash ;\backslash ;\backslash ;\backslash ;4\backslash ;\backslash \backslash -\backslash ;\backslash ;\backslash ;\backslash ;\backslash ;7\backslash \backslash \backslash underline\{-\backslash ;1\_1\backslash ;0\}\backslash \backslash \backslash ;\backslash ;\backslash ;\backslash ;\backslash ;\backslash ;\backslash ;\backslash ;\backslash ;0\backslash end\{array\}$

1. Subtract the ones: Since $11$ is less than $(4+7)(4\; +\; 7)$, calculate instead $11-(4+7+0)=11-11=011-(4+7+0)=11-11=0$ and "memorize" the $11$ at the tens digit.

2. Subtract the tens: $2-(1+1)=2-2=02-(1+1)=2-2=0$