Hello everyone, I’m currently learning Esperanto through the La teorio Nakamura course. I have progressed to chapter 4 - “Mi ne kredas!“ and are stuck at exercise 16.
The question states: “ Please answer with complete sentences, when possible. Write the numbers as words.
En kiu jaro vivas la juna Filipo?”
The question requests users to write a full sentence for each question, but since there are so many ways to write a sentence, I couldn’t figure out what is the true answer nor the intention of the problem.
For example, for the question “ El kiu jaro venas la maljuna Filipo?”, I have answered “ La maljuna Filipo venas el la jaro du mil dek du”, whose format is the same as the sample answer given above, but the whole one is marked incorrect. I have put many tries into the two other questions too, but none of them is correct.
What is the way to solve this problem and others of similar kind?

https://lernu.net/gramatiko/nombraj_vortoj

Eble ĉar multobligo estas sufiksa, kaj aldono plurvorta:

Dek du (10+2)
Sed
Dudek (2×10)

memoryboyvn:
For example, for the question “ El kiu jaro venas la maljuna Filipo?”, I have answered “ La maljuna Filipo venas el la jaro du mil dek du”, whose format is the same as the sample answer given above, but the whole one is marked incorrect. I have put many tries into the two other questions too, but none of them is correct.
What is the way to solve this problem and others of similar kind?

Saluton kaj bonvenon al la kurso!

The course was made before AI, so it works quite strictly when checking answers. The "trick" with written out numbers is, which parts are written together and which not. This Lernu course uses the conventions laid out in Plena manlibro de Esperanta gramatiko (PMEG) which gives the following rule:

• 10–19 are written separatedly: dek unu, dek du, dek tri…
  
• tens and hundreds are written together: dudek, tridek, kvardek… resp. ducent, tricent, kvarcent…
  
• thousands are written separatedly: du mil, tri mil, kvar mil…

Note, you do not normally write unu dek, unu cent or unu mil.

The rationale is that before a thousand can be up to 999, so you not end up writing a more than mouth-full *naŭcentnaŭdeknaŭmil* but a more manageble naŭcent naŭdek naŭ mil for 999 000.

Metsis pravas, ĉar la regulo, kiun mi skribis funkcias nur ĝis 999.

Mil ne funkcias kiel cent, ĉar la multobligo estas farita per la tri lastaj ĉiferoj (aŭ nuloj kiam ili mankas), anstataŭ nur la lasta.

Ducent sed du mil

Jen ĉiuj kombinaĵon kun skribmaniero:
Mil ducent= 1200 = 1000+2×100
Mil cent du= 1102 = 1000 + 100 + 2
Du mil cent = 2100 = 2×1000 + 100
Ducent mil = 200 000 = (2×100)×1000
Cent mil du = 100 002 = 100×1000 + 2
Cent du mil = 102 000 = (100+2) × 1000