I think that it is OK to be corrected when the English spelling is wrong, makes me improve my English writing at the same time, and I get to practise the same exersise one more time, which I do not if it is just a typo. (I too wrote fourty instead of forty). After all: We are learning two languages at the same time, Duo means two! Of course here it is like learning three: Chinese characters, Pin Yin and English at the same time!
I heard "sushi"(ps, I'm obsessed with sushi) and when I saw that it acctually meant, "forty", I almost died laughing. I'm also studying Japanese, which uses the same letters when it comes to numbers but with different pronunciations, so I was very shocked that it would be pronounced that way. This also makes me curious, how do they pronounce "sushi" in Chinese? The life a multilingual is full of mysteries!