I hate the fact that some people think it's not important. I always try and solve things in my head before resorting to calculators, I even set myself complicated ones that can take several minutes just to keep in practice.
An easy example is working out how many seconds in a day. It can be broken down until what might sound complicated is simple.
60 * 60 = ?
6 * 6 = 36
36 * 100 = 3,600 seconds in an hour.
3,600 * 24 = ?
3,600 * 10 = 36,000
36,000 * 2 = 72,000
3,600 * 4 = ?
3,600 * 5 = 18,000
18,000 - 3,600 = 14,400
72,000 + 14,400 = 86,400 seconds in a day.
Always break problems down into something either easy to calculate or something you know by heart like 6 * 6 = 36.
Another tip is when multiplying my 5 it's easier to divide by 2 then multiply by 10. 36,000 * 5 is the same as (36 / 2 = 18) * 1000.
Dividing by 5 is similar, just divide by 10 then multiply by 2. Little tips like that can save a lot of time.