These are the instructions:
Create a formula that shows where an Opportunity is in the pipeline.
Create a formula field that classifies an Opportunity as either “Early”, “Middle”, or “Late”. This formula field should use TODAY() to calculate what percentage of the time between an opportunity’s CreatedDate and CloseDate has passed, and label the opportunity accordingly.
This formula should be on the Opportunity object
This formula should be named 'Opportunity Progress' with the resulting API name Opportunity_Progress__c
This formula should return 'Early' if less than or equal to 25% of an opportunity has passed
This formula should return 'Middle' if between 25% and 75% of an opportunity has passed
This formula should return 'Late' if more than 75% of an opportunity has passed
This formula should reference a helper formula field, also on the Opportunity Object, with the type Percent and the name Percent Completed
Percent Completed should return the percentage of the time that has passed between an opportunity’s CreatedDate and CloseDate

This is the Error:
Challenge Not yet complete... here's what's wrong: 
The 'Opportunity_Progress__c' formula field does not exist or is not of type 'Number'

If the formula is supposed to return "Early", "middle", or "Late", how can it be a number?  it has to be text.  Everything is working as I expect it to, but I can't pass the trial.
Am I missing something?

Sales Managers have asked for an at-a-glance solution to see completeness on leads. Create a helper formula field that looks at 5 key fields on the Lead object and evaluates their completeness, then a second formula field that references the helper formula and returns an image.

>The helper formula field should be on the Lead object with a name of 'Lead Quality Helper' and a resulting API name of 'Lead_Quality_Helper__c'.
>The helper formula should be of type Number.
>The helper formula should evaluate the following 5 fields: Email, Phone, Company, Title, and Industry and return 0 if blank and 1 if not blank. >The formula should then add all the values together to return a total value.
>The image formula should be on the Lead object with a name of 'Lead Quality' and a resulting API name of 'Lead_Quality__c'.
>The image formula should reference the helper formula, and return an image based on the number returned by the helper formula. The helper formula should be of type Text. Note: All of these images are already available in your Developer Edition.

1 = /img/samples/stars_100.gif with alternate text '1 star'
2 = /img/samples/stars_200.gif with alternate text '2 stars'
3 = /img/samples/stars_300.gif with alternate text '3 stars'
4 = /img/samples/stars_400.gif with alternate text '4 stars'
5 = /img/samples/stars_500.gif with alternate text '5 stars'

If none of the fields are filled out, the default should be /img/samples/stars_000.gif with alternate text '0 stars'.
The 'Lead Quality' formula must be added to the Lead Layout page layout.

ok so im having trouble on this challenege on trailhead. im confused what it means by helper formula, like how would we create a helper formular? also how would we create a image formula?

thanks if anyone answers