3. We want to solve using properties;
[tex]\begin{gathered} 5\times23\times2 \\ =5\times2\times23 \\ =5\times2\times(20+3) \\ =10\times(20+3) \\ =(10\times20)+(10\times3) \\ =200+30 \\ =230 \end{gathered}[/tex]It simply involve trying to reduce numbers to their simpliest form to make multiplication and addition easy without using calculator.
like; 21=20+1, 39=40-1 etc.
4. Given;
[tex]\begin{gathered} 8\times51 \\ =8\times(50+1) \\ =8\times50+8\times1 \\ =400+8 \\ =408 \end{gathered}[/tex]