a) We can type letters by using a computer.
Some friends told me that, It's unnecessary to write "type letters"
If we say, " We can type on the computer." it's clear. Am I right?
Generally that's true, particularly if by 'letters' you mean 'a, b, c, d . . .' etc.
But if you mean 'correspondence', it doesn't soumd terribly odd to include the word.
Best wishes, Clive