It's a sense of what numbers are. As a decimal, 1/6 is 0.1666.., so that will mean that 6 times 16 is around 100. How many seconds in a day? It's 60 times 60

It's a sense of what numbers are. As a decimal, 1/6 is 0.1666.., so that will mean that 6 times 16 is around 100. How many seconds in a day? It's 60 times 60 times 24, but 24 is around 25, which is 100/4. So it's 60 times 15 times 100, and 6 times 15 is about 6 times 16, which is 100, so it's 10 times 100 times 100, which is 100,000. But that's too big by about 4% (because we used 25 instead of 24) and another 6% or 7% (from using 16 instead of 15) so it's about 11% smaller. So seconds in a day is about 88,000. All small numbers, few facts, but being comfortable in using them quickly with a side order of sanity checking.