is China sustainable?

Given that China’s yuan is kept artificially low relative to the dollar (~40%), how long can this policy continue? Has this strategy been employed by other countries in the past? If the yuan were to significantly appreciate, how significant would the harm be to both China’s and the U.S.'s economy?

Their Reserve to GDP ratio cannot be maintained, neither has ever maintained in past, it’s already alarmingly high and increasing. Secondly, Chinease economy depends heavily on construction, as of now that’s a systemic risk too, maybe that’s why the real estate bubble isn’t a bubble as it’s the fuel itself. They must be having a exit plan, maybe Chinease Govt may go out on a shopping spree dumping dollars. But who knows, maybe Italian police crack down on Mafia, Italian ports become clean, and Chinease counterfeiting takes the hit.