16 June 2017.  These days about the only math I do is kenken puzzles.  As in sudoku, each digit occurs once in each row and each column. But in kenkens the numbers in each group must satisfy some arithmetic requirement, having the correct sum, product, difference, or ratio. 

I do them in my head - no notes or pencil calculations - so it’s good memory practice.  Currently I do two or three a day.  The NY Times publishes several puzzles a day, some in the paper, more on their website.  This one is from