If the time taken by a randomly selected fifth grader to complete a certain obstacle course is normally distributed with mean value $5$ minutes and standard deviation $1$ minute, the time that separates the fastest $10\%$ from the slowest $90\%$ is a$)7\mathrm{.}33$ b$)2\mathrm{.}67$ c$)3\mathrm{.}72$ d$)6\mathrm{.}28$ e$)$ None of these