I have a 40 amp double pole breaker in my main service panel which feeds a pony box in my attic. A window air conditioner is tripping the 40 amp breaker in the main panel before the 10 amp breaker in the pony box. The 10 amp breaker services two receptacles and one track light. The track light has 4, 50 watt halogens ( not on all the time ) and the receptacle that is not being used by the air conditioner is used rarely. If I use the air conditioner in a different receptacle (not connected to the 10 amp breaker), the 40 amp breaker does not trip. Why is the 40 amp tripping before the 10 amp breaker, and why would it not trip if I use a different receptacle?

Thanks

This is most likely happening because one side of the feeder to the sub-panel is overloaded. When you move to a different receptacle it is probably on the other leg of the panel.

Very little you can except try and balance the load better.