More simply: 
	def multiply(\*numbers)
		numbers.flatten.inject(:\*)
	end
Using \* in the arguments means the arguments will come in as an array. You can call flatten, just in case arrays were passed (making your argument an array of array) to ensure you have a simple array to work with. Flatten won't harm an already flat array. Then you can work with any arguments passed the same way, without need of a conditional.