Does extra mental effort burn more calories?

Which says it goes up by 50%, i.e. an extra 125 calories per hour. He says "But you do not engage that long in such an activity" - maybe he doesn't. But I think most of the people here do.

“There are good, sophisticated studies that show that concentrated mental activity, like doing a difficult multiplication problem in your head, increases the glucose uptake to the brain,” he said. By how many calories? Less than, say, 20 calories of the 300, he estimated."

If it's an increase of 20 calories, then why is the total still 300?