"קמח תפוח אדמה."
Cool! Just yesterday, I learned that in Old English, cucumbers were called earth apples: https://en.wiktionary.org/wiki/eor%C3%BE%C3%A6ppel#Old_English.
Not sure why you got a downvote, As a non-native it seems a legitimate alternative, "flour of potatoes". Google translate uses the singular though. This is a triple construct, and most other flours e.g. almond flour, that are only double constructs use the second noun of the construct in the plural, so it makes sense to use תפוחי. Hebrew speakers, which is preferred?
Dan: In case you missed it, misho104 previously posted a link to this URL:
Since it's a dynamic search-result page, we may get some different images. Just now I saw at least 3 that use תפוח (singular), although the great majority use תפוחי (plural). Also, I saw at least 3 that use the abbreviation with gershayim
Since that's a contraction of 2 words meaning potato, I wonder if it is used to avoid a commitment to spelling the first word as תפוח or תפוחי,
either of which combines with אדמה in the context of potato flour.
Edited with strikeout per info below from Yarden.
The acronyms תפ"א or תפו"א is very common, and is used for either a single or multiple potatoes, in just about any context. In just about any context I can think of, whether it's one potato or many is either obvious or doesn't matter or is meaningless (as with "flour"). The abbreviation is used for convenience, surely not to avoid singular/plural commitment.
To your question of pronunciation of the abbreviated form: it's pronounced fully, /tapuach/tapuchei adama/ (though, since it's so useful and yet so long, you'll often hear /tapcheadama/. Between my wife and me, we say /tapa/, but it's kind of private joke. Probably other people use the same private joke, but I wouldn't try it with someone I'm not intimately familiar with.
EdvntL - the definite form, formally, would be קמח תפוחי האדמה. Generally, there could be a chain of any length of constructs, the ה would come on the last: יצרני מכסי קופסאות קמח תפוחי האדמה.
In spoken Hebrew, however, most people would say הקמח תפוחי אדמה (and היצרנים של המכסים של הקופסאות של הקמח תפוחי אדמה. But not הקמח של התפוחים של האדמה, because קמח תפוחי אדמה is kind of a set phrase.)
In that situation, one can exit out of the lesson and hope that you don't get the same question with the same problem next time.
Before exiting, it would be great to take a screenshot to show the recommended answer that you can use if (when) you do get the same question again; and you can post the photo to the bug-report page on Duolingo website.
When answering the question the second time, be sure to type very carefully.
Here's an informative article to replace the one that FaizalZahid posted, which is now a dead link.