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project_euler/2.rb

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# Project Euler Problem 2: Even Fibonacci Numbers

# https://projecteuler.net/problem=2

# Each new term in the Fibonacci sequence is generated
# by adding the previous two terms.
# By starting with 1 and 2, the first 10 terms will be:
#   1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
# By considering the terms in the Fibonacci sequencewhose values
# do not exceed four million, find the sum of the even-valued terms.

def solve(n)
    a, b, sum = 1, 2, 0
    while a < n do
        sum += a if a % 2 == 0
        a, b = b, a + b
    end
    return sum
end

puts solve(4_000_000)

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