The average age of a family of six members is 30 years. The youngest members of the family are twins aged 10 years. Find the average age of the family just before the birth of the twins.

The earlier average was 30 years.

Total age of 6 members now = 6 x 30 = 180.

Excluding the twins (10 each), the other 4 total = 180 - 20 = 160.

Ten years ago, before the twins, those 4 were each 10 years younger: 160 - 40 = 120.

Average of the 4 then = 120 / 4 = 30.

Method: remove the twins and roll back 10 years.