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She lifted her head, and he could almost see the mental arithmetic "Five whole months!"

They had to be set off one against the other by a process of rapid mental arithmetic.

I opened all the bills and did some quick mental arithmetic.

He looked like a man trying to do a complicated piece of mental arithmetic.

Jack did some mental arithmetic and decided it was evening on the other side of the pond.

Then I did a little mental arithmetic and figured it out.

He looked at his watch and did fast mental arithmetic.

He did quick mental arithmetic and named a large sum.

It was mental arithmetic - to someone who understood circles.

I still didn't have a fix on the mental arithmetic, but the money was clearly huge.