Not if you just remember 3,5,8,13 . You do not need to remember all fibonacci numbers, just the above and rest you can multiple by 2,5,10 and get them

 For me, I think it’s pretty close for the numbers that are right on, and multiplying is way easier for intermediate numbers.50->80 is one beat in my brain, it’s basically immediate because I know the first many Fibonacci numbers.But multiplying by 1.6 is only two beats, it’s “add a half” and then “add a tenth”, each of which come just as automatically as recalling a two Fibonacci numbers. 50->75->80.For the in-between numbers, 1.6 seems way easier. 40->60->64 is much quicker for me than averaging 50 and 80.
 How many km is 4 miles? That's not on the list of numbers.
 4 is the average of 3 and 5, so its km value is the average of 5 and 8 (assuming the Fibonacci series to be a geometric progression as described in the tweet).While I can do this for 4km, I can't for different values like 9km. I've done the multiply-by-1.6 thing often enough by now to be fast enough at it, so I'll likely be sticking to it. This is a cool trick nevertheless.
 Or use them in combination:9 is 8 + 1, so that's 8->13 + 1*1.6 = 14.6
 8 -> 13 and halve it for 6.5?Or any other combination, but using a higher Fibonacci number is going to be more accurate than combining smaller ones.Or in this case, as it's only 1 off a Fibonacci number you can convert that one and add 1.6 without multiplying it by anything (multiplying by the 1 off).
 Just under 7. Or 6.666...Because knowing a km is 3/5 of a mile is more useful. It follows that a mile is 5/3 of a km. From there it's basic math. 4 * 5 is 20. 20 / 3 is 6.666...
 Or 5/8 of a mile if you want a more accurate approximation. Or 8/13. Or 13/21.
 I would solve it mentally as 4 miles = 3 miles + 1 mile.3 miles ~=5km1 mile = 1.6kmSo 5km + 1.6km ~= 6.6km

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