You need 845 ml of a 75% alcohol solution. on hand, you have a 35% alcohol mixture. how much of the 35% alcohol mixture and pure alcohol will you need to obtain the desired solution?
you will need
ml of the 35% solution
and
ml of pure alcohol.

Let be the amount (mL) of the 35% solution you need to use, and the amount (mL) of pure alcohol. Mixed together, you want to end up with 845 mL of solution, so that

For each mL of the 35% solution used, there is a contribution of 0.35 mL of alcohol, while each mL of the pure alcohol solution contributes 1 mL of alcohol. You want to end up with a solution at 75% concentration, so that

We can solve easily by substitution:

Then

So you will need 325 mL of the 35% solution and 520 mL of pure alcohol.

a) one student is chosen from classroom a, then a student is chosen from classroom b.  

step-by-step explanation:

independent events are ones in which the probability of one event does not affect the probability of the second event.

one student is chosen from classroom a; this does not affect the chances of choosing a student from classroom b.   this makes these independent events.

when one card is chosen from a standard deck and then set aside before a second card is chosen, the probability of the second card being chosen changes.   this makes them dependent events.

when one student is chosen from classroom a and then a second student is chosen from classroom a, the probability of choosing the second student changes.   this makes them dependent events.

when a letter is chosen from the alphabet and then a second letter is chosen, the probability of choosing the second letter changes.   this makes them dependent events.