A school runs Monday to Friday. It closes on the orange holidays, and when a holiday falls on a Sunday the next Monday is given off instead. How many school days are there in the month shown?

August 2030 starts on Thursday and has 31 days, so Thursday, Friday and Saturday occur 5 times. Monday-to-Friday days: 5 (Thu) + 5 (Fri) + $4\times 3$ (Mon, Tue, Wed) = 22. Holidays: 15 August (Thursday) is off; 18 August is a Sunday, so Monday 19 August is off instead. 22 − 2 = 20. (21 forgets the substitute Monday; 22 ignores both holidays.)