For me it was estimation that unlocked the magic of mathematics. What's the fastest way you can get close enough to the correct answer that the innaccuracy doesn't matter?

For example, in salary negotions if you need to convert between hourly and annual salary take the annual value, drop 3 zeros and divide in two and you have the hourly. Or in reverse, take the hourly, multiply by 2 and add 3 zeros for the annual. It gets you close enough that you can know you're talking about for salary negotiations, budgeting, etc. and you can let the computer calculate out the exact conversion for payroll and tax withholding purposes