Tourville squinted against his cigar smoke while he did the mental math.

That was before he started his first salon. I did some mental math.

Using mental math, I calculated that that was over four hundred miles per tank.

Irina does mental math, and calculates she'll be forty when the baby is nineteen.

He did a little mental math.

I did some mental math.

He did some mental math.

Hope glanced at the clock on the kitchen wall and did some mental math, taking into account the horsepower in Gideon's Challenger.

After this came Arthur Benjamin's mental math session, which got rave reviews from people walking out just as I finally arrived in San Diego.

"I dunno," Amos replied, his lips compressing as he did the mental math.

"I'm glad you did," Ranjit said slowly, but his eyes had darkened as he did the mental math.