The number of inmates in state and federal prisons jumped 42,862 during the first half of 1990 as the nation's prisoner population reached a record 755,425, the Justice Department said. The 6% increase during the six-month period was only slightly less than the record 46,000 jump during the first half of 1989, said a Bureau of Justice Statistics study of state and federal prison populations. Incarcerations ran at a record annual increase of 80,000 prisoners from June, 1989, to June of this year.