$a)$

$16\cdot 20=320\left[16\cdot 2\cdot 10=32\cdot 10=320\right]$

$17\cdot 30=510$

$24\cdot 50=1200$

$40\cdot 60=2400\left[\underline{4}\cdot 10\cdot \underline{6}\cdot 10=\underline{24}\cdot 100=2400\right]$

 

$b)$

$150\cdot 40=6000$

$280\cdot 20=5600$

$240\cdot 300=72000\left[\underline{24}\cdot 10\cdot \underline{3}\cdot 100=72\cdot 1000=72000\right]$  

$160\cdot 400=64000$  

 

$c)$

$120\cdot 500=60000$

$210\cdot 300=63000$

$700\cdot 200=140000$

$300\cdot 120=36000$