1000 parts arrive form a maufacturer in 2 batches. Each unit of the first batch cost 39.00 while each unit of the second batch costs 59.00. If the total cost was 55,000, how many units were in each batch?

The answers are 200 and 800

How would I explain this?

Set up a system of two equations with two unknowns.

x=number in first batch and y=number in second batch.

x+y=1000

39x+59y=55000

Solve the little system by substitution or elimination or whatever method you prefer.