With a calculator this can be done by doing 420*1.11.
Without a calculator, it is also very easy, and is done by finding 10% of 420 which is 420/10 which is 42. So 42 = 10% of 420. And 1% of 420 is 4.2. 1%+10%+100% = 4.2+42+420 =466.2.
As your older brother, I've been where you are—maybe not exactly, but close enough.
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Oh, dude, math time! So, to increase 420 by 11%, you just multiply 420 by 0.11 (which is 11% as a decimal), then add that to 420. That gives you a grand total of 466.2. Like, whoa, it's that easy!