Calculate nth prime in ruby

This program calculates nth prime in ruby

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def isDivisibe(candidate, by)
  candidate % by == 0
end

def isPrimeDivisible(primes, candidate, count)
  for e in (0...count)
    return false if Math.sqrt(candidate) < primes[e]
    return true if isDivisibe(candidate, primes[e])
  end
  false
end

def countPrimes(size)
  primes = Array.new(size)
  count = 0
  candidate = 2
  while count < size
    unless isPrimeDivisible(primes, candidate, count)
      primes[count] = candidate
      count += 1
    end
    candidate += 1
  end
  primes[size - 1]
end

for count in Array.[](25_000, 50_000, 100_000)
  result = countPrimes count
  puts "#{count}th prime no is #{result}"
end