1 3/7 * с + 7/8 = 1 3/8
1 3/7с = 1 3/8 - 7/8 (1 3/8 - 7/8 = 11/8 - 7/8 = 4/8 = 1/2)
1 3/7с = 1/2
с = 1/2 : 1 3/7 (1/2 : 1 3/7 = 1/2 : 10/7 = 1/2 * 7/10 = 7/20)
с = 7/20