Just graph it, that’s way too much work to do mentally.
Answer: (no promises this is exactly right)
-2r+6
Step-by-step explanation:
−3r+9+r+−3
=
(−3r+r)+(9+−3)
=
−2r+6
The 70th percentile is the same as the cutoff recovery time

such that every point above this time falls in the longest 30%, i.e.

Transform to the standard normal distribution:

where

is the z-score corresponding to the cutoff time

, which is approximately

. Solve for

: