"A chemist is using 386 millilitres of a solution of acid and water."
This sentence implies that the solution only contains acid and water.
Now, we are told:
"12.4% of the solution is acid."
What is 12.4% percent though, as a decimal?
Well, 12.0% is actually equal to 0.12 as a decimal. 0.4% is actually 1/2 of 1%, which in turn is 1/2 of 0.010 which is 0.005 as a decimal.
So, we now know that 12.4% is actually equal to 0.124 as a decimal.
Let's multiply 0.124 by 386 to get our answer.
0.124 x 386 = 47.86 (rounded to the nearest tenth)
There are 47.86 millilitres (to the nearest tenth) of acid in the solution.