Final answer:
The new depth of the submarine is -40 11/15 km. To find the new depth of the submarine, we need to add the additional depth it descended to its initial depth.
Step-by-step explanation:
The initial depth is -40 ⅔ km, and the additional depth is 10 ⅕ km. To add these values, we need to convert them to the same fractional form. -40 ⅔ km is equivalent to -40 2/3 km, and 10 ⅕ km is equivalent to 10 1/5 km.
Adding -40 2/3 km and 10 1/5 km, we get -40 2/3 + 10 1/5 = -40 11/15 km. Therefore, the new depth of the submarine is -40 11/15 km.