Yellow cells take the total of four numbers, so lie in the range 10 to 30 inclusive

One pink cell is to have a number from 1 to 9

Then the Solve button will solve the puzzle

One pink cell is to have a number from 1 to 9

Then the Solve button will solve the puzzle

Designed and written on a flight as an experiment in "enough design up front"

Jonathan Clarke, February 2014