Laura's brow furrowed as she did the mental math.

After some mental math Aiah comes up with the exact figure: 317.

So it's around 1/1,000,000th the mass of the moon (I always trip over decimal places when trying to do mental math)?

A little mental math allows interim averages, easiest at 1/4 (15 unit) and other integer values.

She did a brief bit of mental math and realized that at the moment, Guild scale for four hours was about $500 per hour.

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

Joe did a bit of quick mental math and came up with a figure far too high to believe.

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

This algorithm meets the Vedic ideal of mental math with one line notation.

The lawyer now had to do a bit of mental math.