Multiplication and Number Bases

$$2321_5\ \times\ 102_5$$  

The answer to the expression above would have __ digits.

$$534_{6\ }\times\ 43_7$$  

Mary says you can work out the expression above. Sam says it's not possible.

Who do you agree with and why?

What's missing from this equation to make it accurate?

$$56_{7\ }\times\ 40_7\ =\ 3230$$  

(a)  

What is the value of $$32_4\ \times\ 20_4?$$  

$$441_5\ \times\ 34_5\ =\ 33144_5$$  

$$212_3\ \times\ 21_{2\ }=\ 1\Delta222_3$$  

Which number should replace the symbol above to make the equation true?