Если ваши данные такие:
x = structure(list(Name = structure(c(4L, 1L, 3L, 2L, 5L), .Label = c("Alan",
"Ana", "Hector", "Helen", "Sophie"), class = "factor"), age = c(12,
14, 15, 11, 16), height = c(1.2, 1.4, 1.25, 1.02, 1.4), speed = c(40L,
75L, 80L, 81L, 50L)), class = "data.frame", row.names = c(NA,
-5L))
Надеюсь, я правильно понял цифры:
Name age height speed
1 Helen 12 1.20 40
2 Alan 14 1.40 75
3 Hector 15 1.25 80
4 Ana 11 1.02 81
5 Sophie 16 1.40 50
Затем выполните:
x[diff(x$speed)<30,]
Name age height speed
2 Alan 14 1.40 75
3 Hector 15 1.25 80
4 Ana 11 1.02 81