Ok I don't know if you will see this, but I just realized that the answer I gave you was probably wrong because I forgot that some places use " . " as a seperator for thousands, not a decimal. In the rest of this post, I will just put spaces to separate thousands to avoid confusion.
actual area of the lake = 18 000 000 square meters
actual area of the lake = 18 000 000 / 10002 square kilometers
actual area of the lake = 18 000 000 / 1 000 000 square kilometers
actual area of the lake = 18 square kilometers
map area of the lake = 18 / 22 square cm
map area of the lake = 18 / 4 square cm
map area of the lake = 4.5 square cm <---(That " . " is a decimal)