In the Journal of Abnormal and Social Psychology, Feather describes a 'insoluble' task --still so complex that the subject is not aware of it. Following, the complete reference:
Journal of Abnormal and Social Psychology
1961, Vol. 03, No. 3, 552-561
THE RELATIONSHIP OF PERSISTENCE AT A TASK TO EXPECTATION
OF SUCCESS AND ACHIEVEMENT RELATED MOTIVES1
N. T. FEATHER
Best, C. Rodeiro