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Correct.

Легко. :) Кошка размером с книгу.

Я написал... (perf. verb, the action has finished)
История means a short story, an event which happened to somebody and told to another person. I...

Я написал ручкой - lit. I wrote by pen like I came by train (instrumental, no need preposition here)
Я иду с ручкой - you cannot say I am going by pen, so here is c + instrumental.

I tore pages...