I am supposed to be changing the setInterestRate function so that it prompts for the current rate and this rate is then assigned to var interestRate. To confirm this, the function displayInterestRate is called and it writes the value to output window. I will need to perform some math on the the setInterestRate variable. I have highlighted the part I need help with (in red).

Here is the code I've got so far:
var interestRate = 0;
var response;
var amountIn, interest;
function twoDPs(anyNumber)
{
return Math.round (100 * anyNumber) / 100
};
function setInterestRate(interestRate)
{
window.prompt('Enter the interest rate', '') // Insert your code here
};
function displayInterestRate(interestRate)
{
document.write('Current interest rate is ' + interestRate + '%')
};
function calculateInterest(numberOfPounds)
{
// Insert your code here
};

// Insert your code here
// Main program starts here

setInterestRate(interestRate);

displayInterestRate(interestRate);

document.write('<BR><BR><BR><BR>');
// The following code may be changed
response = window.prompt('Do you want to calculate the interest on an amount of money (Y/N)? ','');
while(response == 'Y')
{
amountInPounds = window.prompt('Enter amount in Pounds ','');
interest = calculateInterest(amountInPounds);
document.write('£ '+ amountInPounds + ' attracts £' + interest + ' interest');
document.write('<BR><BR><BR>');
response = window.prompt('Do you want to calculate the interest for another amount (Y/N)? ','')
}

I'm not sure what the problem you're having is, but I do notice that in setInterestRate you ask for the rate, but then you don't do anything with it. I would assume you want

InterestRate = window.prompt('Enter the interest rat', '');

and then return InterestRate before leaving the function.

Your function call might then be

rate = setInterestRate(InterestRate);

so you'd have that value outside the funtion which you can manipulate.

Is that what you're looking for?

I got the prompt part sorted, so it asks for current rate, so I put in the rate, but then when it displays it, it just states that the current rate is 0%. It doesn't display what I entered. Does this make sense?

And yes, I think that is the sort of thing I am after. I have only just learned about functions, so am struggling a bit with this.

That makes sense. I think the issue is that while you're asking the user for input you're not doing anything with that input once you get it. You want to set a variable with the user input.

Also variable you create in a function are only known by that function. That's why you need to return the variable.

I think the code I have in my post above is what you want or something like what you want.

Thank you. Got it all sorted now.

Cool. Glad to help if I did. And if not them I'm just glad you got it working.