pjc

upto = 20
 
def prime? x
  (2..x-1).each { |y| return false if x % y == 0 }
  true
end
 
# For speed reasons we want to find max possible increment
def increment upto
  a = 1

pjc

def palindrome? x
  y = x.to_s
  checks = y.length / 2
  y[0..(checks-1)] == y[-checks..-1].reverse
end
 
def divideable? x
  999.downto(100).each { |y| return true if x % y == 0 && (x / y).to_s.length == 3 }
  false
end

pjc

# We make array of prime numbers with modulus 0 as long as product_sum < n
 
def prime? n
  (2..(n-1)).each { |x| return false if n % x == 0 }
  true
end
 
n = 600_851_475_143
a = []
product_sum = 1

pjc

# Create Fibonnaci Array
a = [1,2]
upto = 4_000_000
while a[-2] + a[-1] < upto
  a << a[-2] + a[-1] 
end
 
# Create sum of even Fibonnaci numbers
sum = 0
a.each { |x| sum+= x if x.even? }

pjc

y = 0
(1..999).each { |x| y = y + x if (x % 3 == 0) || (x % 5 == 0) }
puts y