Obliczamy ile wynosi każdy z podanych iloczynów. 

$3\cdot 40=120\left[3\cdot 4\underline{\underline{0}}=\underset{3\cdot 4}{\underbrace{12}}\underline{\underline{0}}\right]$  

$12\cdot 10=120\left[12\cdot 1\underline{\underline{0}}=\underset{12\cdot 1}{\underbrace{12}}\underline{\underline{0}}\right]$      

$100\cdot 12=1200\left[1\underline{\underline{00}}\cdot 12=\underset{1\cdot 12}{\underbrace{12}}\underline{\underline{00}}\right]$  

$30\cdot 40=1200\left[3\underline{\underline{0}}\cdot 4\underline{\underline{0}}=\underset{3\cdot 4}{\underbrace{12}}\underline{\underline{00}}\right]$  

$3\cdot 10\cdot 4=120\left[3\cdot 1\underline{\underline{0}}\cdot 4=\underset{3\cdot 1\cdot 4}{\underbrace{12}}\underline{\underline{0}}\right]$  

$3\cdot 4\cdot 100=1200\left[3\cdot 4\cdot 1\underline{\underline{00}}=\underset{3\cdot 4\cdot 1}{\underbrace{12}}\underline{\underline{00}}\right]$  

$3\cdot 400=1200\left[3\cdot 4\underline{\underline{00}}=\underset{3\cdot 4}{\underbrace{12}}\underline{\underline{00}}\right]$  

Iloczyny równe 120 to: 

• 3٠40
• 12٠10
• 3٠10٠4

Iloczyny równe 1200 to: 

• 100٠12
• 30٠40
• 3٠4٠100
• 3٠400