The question you opened
Four weeks of croissants
A cafe sells 24 croissants a day, seven days a week.
How many croissants is that in four weeks?
672 croissants
Four weeks is 28 days. Read 24 times 28 as 24 times 30 minus 24 times 2: 720 minus 48 gives 672.
A cafe sells 24 croissants every day of the week. Over four weeks that is 672, and there is a pleasant way to get there without paper. First collapse the time. Four weeks of seven days is 28 days. Now the whole question is 24 times 28, one multiplication rather than two. The nice route is to round the awkward number up to a friendly one and then take back what you added. Twenty eight is two short of thirty.
So compute 24 times 30, which is 24 times 3 with a zero on the end, giving 720. Then take away the two extra lots of 24, which is 48. And 720 minus 48 is 672. Rounding up and subtracting beats rounding down and adding for most people, because subtraction from a round number is easier to hold in your head than an addition that carries. Try the other direction and you will feel the difference: 24 times 25 is 600, plus 24 times 3 which is 72, giving 672.
Same answer, slightly more to remember. A third route is doubling, which suits numbers like 28 that halve cleanly. Twenty four times 28 is 24 times 4 times 7, so 96 times 7, which is 672. Choose whichever pair of factors gives you numbers you already know. The habit underneath all three is the same: never multiply the number you were given if a friendlier number sits nearby. Thirty, twenty five, and doubling chains are all easier than twenty eight.
One practical note on the setup. Weekly rates hide a trap in the other direction, because a month is not four weeks. A month averages about 30.4 days, so monthly sales here would run near 730 rather than 672. When a question says four weeks it means 28 days, and when it says a month it means something else. Read which one you were handed before you reach for the arithmetic.
Technique: Round up, then subtract the extra