On a wall calendar, four dates form a small square block: two dates side by side in one week, together with the two dates directly beneath them in the following week. The four dates add up to 80. What is the smallest of the four dates?

Call the smallest date d. The date beside it is d + 1, and the two beneath them are exactly one week later, that is d + 7 and d + 8. Adding gives 4d + 16 = 80, so 4d = 64 and d = 16. The block is 16, 17, 23, 24, and those really do add to 80. The traps: 20 is 80 ÷ 4, which would be right only if the four dates were all equal; 17 is the date beside the smallest one; 23 is the date directly beneath it. Both of those answer for the wrong corner of the block.