Calling all Maths teachers!

What are some of your all-time great tasks for secondary Maths?

Ones that could have their place in any curriculum, one that should be studied in PGCE courses, ones that started whole new genres of tasks...
... or just ones you really like.

(see some of mine below)

7% of each 100 is 7. 7% of the remaining 50 is half that, or 3.5. So 7+7+3.5 =17.5

Thank you for sharing. My answer to this question changed over the years. In fact, my progression seemed to follow the order of the responses you laid out!

I’d look for as many 100’s as possible because I know 4 x 25 =100. Essentially, how many 4’s are in 37?

37= 36+1
= 4(9)+1
This gives me 9 100’s and one more 25 so 925.