1. The following readings were taken with a Dumpy level and a 4 m levelling staff on a continuously sloping ground at 30 m intervals : 0.680 m, 1.455 m, 1.855 m, 2.330 m, 2.885 m, 3.380 m, 1.055 m. The R.L. of the fourth point was calculated to be 79.100 m. The R.L. of the point that was read 0.680 m is