BILLERICA - Helen G. (Devitt) MacCormack, 79, died yesterday, May 16, at the Lahey Clinic Medical Center in Burlington.She was born in Boston, on Aug. 16, 1923, a daughter of the late John B. and Margaret (Berrigan) Devitt. She had lived in Milton and then Bedford for 19 years before moving to Billerica 32 years ago. For 22 years, Mrs. MacCormack worked in the accounting department at MITRE Corp. in Bedford, retiring 14 years ago. Survivors include three sons, Hugh MacCormack of Beverly, and John MacCormack and Robert MacCormack, both of Billerica; two daughters, Susan McLaughlin of Hamilton and Karen MacCormack of Manchester, Mass.; seven grandchildren; a great-granddaughter; and several nieces and nephews.