Answer:
C. 40mm
Explanation:
To solve for leg a:
Since the hypotenuse is 50mm and leg a is 30mm we can use the following formula below.
a =

will be
because c = hypotenuse
will be
because b = leg b
Now that we have replaced our variables we can answer the equation:

50 × 50 = 2500
30 × 30 = 900
Now we subtract:


square root of 1600 = 40
a = 40