All of these problems are similar (ie they use the same methods to find the answer). So I'll do the first one to get you started.
# 1
"a conservationist catches 193 trout, tags them and throws them back into the lake. Later, 24 trout are caught, 12 of them are tagged. How many trout are in the lake?"
It turns out that we have the following ratio:
Number of Trout Number of Trout
----------------- = -----------------
Number Tagged Number Tagged
Now simply fill in the information to get
Multiply both sides by 193
Multiply
Simplify
So our answer is which means that there are 386 trout in the lake.