yield return WaitForSeconds not working

[ToastHatter]

i have been trying to make a game in unity, and i have tried to add a power up function to it, i tired using the yield return WaitForSeconds function but it wont work, here is the section of code that doesn’t work

	}
}
IEnumerator Powerup(){
	if (count4 == 1) {
		speed = 20.5f;
		powerText.text = "SPEED ORB";
		yield return WaitForSeconds ("30");
		speed = 10.5f;
		count4 = 0;
		powerText.text = "No Power Up";
	
	}
}

}

when i go into the editor it gives the error of Expression denotes a type', where a variable’, value' or method group’ was expected

here is the full code just in case there is an error somewhere else;

using UnityEngine;
using UnityEngine.UI;
using System.Collections;
using UnityEngine.SceneManagement;

public class StoryPlayer : MonoBehaviour {

public float cubes;
public Text countText;
public Text winText;
public Text powerText;
public Text lifeText;
public Text crewText;
public Text timer;

private Rigidbody rb;
private int count;
private int count2;
private int count3;
private int count4;
private int count5;
private float speed;

void Start ()
{
	rb = GetComponent<Rigidbody>();
	count = 0;
	count2 = 0;
	count3 = 3;
	count4 = 0;
	count5 = 200;
	speed = 10.5f;
	SetCountText ();
	SetCrewText ();
	SetLifeText ();
	winText.text = "";
	powerText.text = "No Power Up";
}

void FixedUpdate ()
{
	float moveHorizontal = Input.GetAxis ("Horizontal");
	float moveVertical = Input.GetAxis ("Vertical");

	Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);

	rb.AddForce (movement * speed);
}

void OnTriggerEnter(Collider other)
{
	if (other.gameObject.CompareTag ("Pickup")) {
		other.gameObject.SetActive (false);
		count = count + 1;
		SetCountText ();
	}

	if (other.gameObject.CompareTag ( "Crew")){
		other.gameObject.SetActive (false);
		count2 = count2 + 1;
		SetCrewText ();
	}
	if (other.gameObject.CompareTag ( "Foe")){
		other.gameObject.SetActive (false);
		count3 = count3 - 1;
		SetLifeText ();
	}
	if (other.gameObject.CompareTag ("Speed")) {
		count4 = 1;
		StartCoroutine(Powerup ());
		while (count5 >= 0)
			other.gameObject.SetActive (false);
		if (count5 == 0)
			other.gameObject.SetActive (true);

	}
		
}

void SetCountText ()
{
	countText.text = "Cubes: " + count.ToString();
	if (count >= cubes && count2 >= 1)
	{
		winText.text = "YOU WIN!";
		Application.LoadLevel ("Chapter");

	}

}
void SetCrewText()
{
	crewText.text = "Crew: (" + count2.ToString () + "/1)";
}
void SetLifeText ()
{
	lifeText.text = "Life: (" + count3.ToString () + "/3)";
	if (count3 <= 0) 
	{
		winText.text = "YOU LOSE";
		Application.LoadLevel ("Chapter");

	}
}
IEnumerator Powerup(){
	if (count4 == 1) {
		speed = 20.5f;
		powerText.text = "SPEED ORB";
		yield return WaitForSeconds ("30");
		speed = 10.5f;
		count4 = 0;
		powerText.text = "No Power Up";
	
	}
}

}

if any one can help me fix this or even suggest an alternate method it would be greatly apreaciated

[Eric5h5]

You can’t use a string in WaitForSeconds, only a number.

[jamesbice1]

this website should help:
Unity - Scripting API: WaitForSeconds
also when you assign how many seconds you are waiting, you do not put “”
it should look like this:

	yield return WaitForSeconds(1);