How could thinking about 90 tens help you find 1901-297?

Oh, dude, thinking about 90 tens is like thinking about having 900 of something, right? So, if you have 1901 and you subtract 297, it's like taking away 300 from 1900, which is 1600, and then adding back 3, which gives you 1603. Easy peasy, lemon squeezy!