Jen ate 18 jujubes.
To find the answer to this problem, you should work it backwards.
Kyle had 18 jujubes in his possession before he gave some to Kim. Since Jen ate half and gave half to Kyle, 18 is half of the number that Jen had total. 18 divided by 1/2 will be 36. Jen had a total of 36, but she ate 18. And since Sam gave Jen 36, which was half of what he had, Sam had a total of 72 jujubes.
