def calculate(*array)
  calculation =  array.last
  if calculation.is_a?(Hash)
    numbers = array.select{ |i| i.is_a? Integer}
  else
    calculation = "add"
    numbers = array
  end
  if calculation = "add"
    add (numbers)
  else
    subtract(numbers)
  end
end

calculate(4, 5, add: true)

Error message :

TypeErrorArray can't be coerced into Fixnum