$a)$

$23\cdot 10=230$

$10\cdot 34=340$

$100\cdot 9=900$

$15\cdot 100=1500$

$b)$

$17\cdot 10=170$

$10\cdot 33=330$

$7\cdot 100=700$

$100\cdot 12=1200$

$c)$

$9\cdot 2\cdot 10=18\cdot 10=180$

$7\cdot 8\cdot 10=56\cdot 10=560$

$2\cdot 9\cdot 100=18\cdot 100=1800$

$6\cdot 8\cdot 100=48\cdot 100=4800$