It took me a little while to figure out how I'd actually say this in English after translating it as "four is twice two" (which is accepted, and was the least awkward/nonsensical way I could think of at first). What I'd really say is "four is two times two", rather than using the word "double" or "twice". It's not a word-for-word translation, but speakers of American English, at least, just wouldn't say it like the French version.
It seems silly that in introducing 'le double' that Duo Lingo does not accept "four is two doubled", which is correct English - even if rarely used. To force the use of 'double' then accepted translations should be: Four is two doubled. Two doubled is four. Four is the double of two. Double two is four (or Twice two is four - which would be more common) Four is twice two (not Four is double two - not sure why but double doesn't work that way) "Doubled" is not that rare a word. For example "He doubled down on his bet." It does imply a past tense which "le double" does not, but French and English use of tense is not the same in many areas (much to my frustration!). :-)
Hi Mike, Ya, I was just curious.. At one point, DL did show a percentage, but now I cannot find that. Inspiration for you.. The former Prime Minister of Singapore, Lee Kuan Yew, took up studying Chinese and continued to do so into his 90's. There is a fair amount of Chinese speakers in Singapore, and he wanted to be able to address them in Chinese. He is known as the father of Singapore, having developed it from mud flats and a third world country, into the thriving financial metropolis that exists today. Yet all the while he continued to study his Chinese. So I know I have the time to study as well.