I approached this problem in a weird but I got the answer so idk if it's the most reliable way;
1) I Knew that S1 has 20% of salt and S2 has 30% of salt. when added with each other the new soln has 24% salt so:
20----24------30. as we can see that 24 is closer to 20 so the new soln contains more of S1 and ratio of S1 and S2 in the new soln, let's say Y is S1:S2 = 6:4.
2) We know that 4 litre of S2 was added. so by going with the current Ratio we'd have 4 litres of S2 and 6 litres of S1.
3) Problem solved? Not so fast. We know that before adding S2, S1 was heated to lose half of its volume. So, the initial would have been twice the volume of S1 currently which is 6. so S1 (initially) would have been 12. so, 6 ounces of it were heated.
Carcass,
Greenlight Test Prep could you evaluate and verify (if true) my approach?