The sorted marks are 64, 78, 78, 85, 92…

Looking for $85$. The places are $0$ to $4$. Step 1: the middle is place $2$, which holds $78$. $85$ is bigger, so keep places $3$ to $4$. Step 2: the middle is place $3$, which holds $85$. Found, after 2 steps.

Looking for $70$. Step 1: place $2$ holds $78$, which is bigger than $70$, so keep places $0$ to $1$. Step 2: place $0$ holds $64$, which is smaller, so keep place $1$. Step 3: place $1$ holds $78$, which is bigger, and nothing is left. Not found, after 3 steps.