に (per unit/rate)

Summer in Nagoya is so hot I always change clothes two or three times a day.

I study English on average 2 hours a day.

I make a point of writing to my mother once a month.

She writes me a letter once a month.

She sends a letter to her pen friend in London twice a month.

I earn 100 euros a day.

The train ran at a speed of 500 miles per hour.

I brush my teeth twice a day.

I write to my mother once a month.

On average, I go to the movies twice a month.