Below is an instruction that describes a task. Write a response that appropriately completes the request in Thai. Please answer in Thai.

### Instruction:
In the jar, there are 7 blue marbles, 11 red marbles, and some yellow marbles. If the chance of randomly picking a yellow marble is 1/4, how many yellow marbles are there?

### Response:
 กำหนดให้ y เป็นจำนวนลูกแก้วสีเหลือง
มีลูกแก้วทั้งหมด 7+11+y=18+y ลูก
y/(18+y)=1/4
4y=18+y
3y=18
y=6