PE Exam Practice Question

Hi, I am having trouble understanding the answer to this practice question. Can someone please show a step-by-step solution? I don't understand how the effective stress is 2760.6 psf. I am getting 2794.4 psf. Thanks in advance.

https://preview.redd.it/iuxjdfai0vng1.png?width=1264&format=png&auto=webp&s=ec9ff3b6dda5d571ca24df0f85b1faee61cb9e89

https://preview.redd.it/kf3j1dzi0vng1.png?width=708&format=png&auto=webp&s=a8dd5b7b475f203c0acc6fff9b2d52871cf18968