450 is 1 std deviation from 500 and 600 is 2 std deviations. So what you wanna do is for plus and minus 1 std deviation the probability is something like ~67% but you want halve of that. so for the 450 bit its 33%.
Then for 2 std deviations plus minus its ~90ish%, but you want the the plus side, so you divide ~95ish% by 2 and get 48ish%.
so then you add 33% and 48% to get B
Also my standard deviations % are off the top of head so you will wanna check them :)
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