You just need to do the opposite in order to find the total value since you divided the treasure up we need to multiply the treasure “back”.
3 buried bars is equal to 1/3 of the remaining treasure. Multiply by 3 to get 3/3 or 100% of buried and crew portion. So we now have 9 bars. Then multiply by 2 to get 100% of what was given to bill, crew and buried. We now have 18 bars. Then we know that we took 1/4 for ourselves so that means 18 bars represents 3/4 of the treasure. So when we look at factors of 18 we find that 3 x 6 = 18 and 4 x 6 = 24 so we conclude that we had 24 bars of treasure before the split.
