Sum up data

Hi, 

I have this field (surv_quantity) but a job level in my table surv_jobs

I have this field (surv_out) at a sub job level in my table surv_sub_job

What I need to do it take the surv_jobs.surv_quantity - surv_sub_jobs.surv_value

The problem I have is that the value surv_quantity double counts as it see 2 jobs as it has 2 sub jobs. 

Example-

Surv_id=10

surv_quantity I have a total value of 30,000

However, if I look at the detail whereby job 

Surv_id=10

Surv_sub_id=12 value=30000

Surv_sub_id=23 value=30000

By having 30000 twice, it's scewing the numbers as it thinks for that job, the value is 60000 when in actual fact, its 30000. 

Any idea how I can get around this please so it only takes the one value?