Happy 4/20 Day 2018

Computer Science Level 2

Let W E E D ( x ) = 4 x + 20 WEED(x) = 4x + 20 and W E E D y ( x ) WEED_{y}(x) be a recursive function such that W E E D ( W E E D ( . . . WEED(WEED(... is iterated y y times. What are the last 33 digits of W E E D 420 ( 420 ) WEED_{420}(420) ?


The answer is 618590466360255786349832360864420.

Stefan Popescu by Stefan Popescu , 16, USA

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3 solutions

Zeeshan Ali
Apr 28, 2018 
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def WEED(x, y=1):
    if y==1:
        return 4*x+20
    return 4*WEED(x, y-1)+20

print (int(str(WEED(420, 420))[-33:]))


1
 
618590466360255786349832360864420

2 Helpful 0 Interesting 0 Brilliant 0 Confused
Giorgos K.
Apr 25, 2018

here is a M a t h e m a t i c a Mathematica code

Nest[4#+20&,420,420]~Mod~(10^33)

returns 618590466360255786349832360864420 618590466360255786349832360864420

1 Helpful 0 Interesting 0 Brilliant 0 Confused
Stefan Popescu
Apr 20, 2018

There's no easy way to do this, so the best way is to write a computer program which will do what would normally take you weeks to calculate. This is a program written in Python 3.4. It can be seen that the full value of n n is a whopping 256 digits!

1 Helpful 0 Interesting 0 Brilliant 0 Confused

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