Solving large multiplications in my head What would be the best approach to solve 73 x 42 in my head?
I started with 70 x 40 and then 3 x 40 and combined, but at this point I forgot what I had done and ended up getting lost and not figuring it out.
Is there a good method for solving multiplication as such in my head?

Many mental calculators do mutliplications from left to right this way :
\begin{array}{r}
73\\
\times\; 42
\end{array}
\begin{array}{c l}
\text{'cross' computation} &\text{ partial result}\\
7\cdot 4=28 & 28\\ 
7\cdot 2+3\cdot 4=26 & 306\\
3\cdot 2=6 &3066
\end{array}
Other example :
\begin{array}{r}
237\\
\times\;543
\end{array}
\begin{array}{c l}
\text{'cross' computation} &\text{ partial result}\\
2\cdot 5=10 & 10\\ 
2\cdot 4+3\cdot 5=23 & 123\\
2\cdot 3+3\cdot 4 +7\cdot5=53 &1283\\
3\cdot 3+7\cdot 4=37 & 12867\\
7\cdot 3=21 & 128691\\
\end{array}
To compure the square of a number use $a^2=(a+b)(a-b)+b^2$.
For example $$78^2=80\cdot 76+2^2$$
Other methods and examples in this MSE thread.
