There are a few options to achieve this.
File upload
The first option is to use the test setting "file upload". You can find this option in the Teacher environment, when you're scheduling a test.
You can allow students to upload a file once per test, or per test question. This can be a PDF, an Excel-file or indeed a photo of their calculations.
To enable it, go to the test settings- and under Tools, find "Students can upload files during the test".
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Additionally, it's also possible to upload files by scanning a QR code on the screen, which will allow the students to upload their files directly from their phone (read more here).
How the students will see it:
Students will get the extra Uploads button either on the test level, or on every exercise on the test (depending what you chose on the settings).
Add a note
The second option is to use the test setting "note". You can allow students to add a note to each question. This means that in addition to their regular input field, where they can input the final answer, they have an additional input field where they can mix text, formulas and even drawings.
To allow students to add a note, go to the test settings and under Tools choose "yes" for: Students can add a note to each question.
How the students will see it:
Students will get the extra Note button with every exercise on the test.
Once they click on the Note a text editor will show up.
! Both uploaded files and notes will have to be checked and graded manually by the teacher. You can see the uploaded files and notes in the grading view.