Delete rows based on previos rows

General
10

This is becoming a little frustrating to watch, you are supposed to ask your questions providing a proper REPRoducible EXample (reprex) illustrating your issue.

Like in this answer

library(tidyverse)

sample_df <- data.frame(
          ID = c(1, 1, 1, 1, 1, 2, 2),
       dosis = c(10, 0, 10, 0, 10, 10, 0),
        days = c(100, 70, 100, 35, 700, 50, 12)
)

sample_df %>% 
    group_by(ID) %>% 
    mutate(flag = if_else(dosis == 0 & days >= 30, TRUE, NA)) %>% 
    fill(flag, .direction = "down") %>% 
    filter(is.na(flag)) %>% 
    select(-flag)
#> # A tibble: 3 x 3
#> # Groups:   ID [2]
#>      ID dosis  days
#>   <dbl> <dbl> <dbl>
#> 1     1    10   100
#> 2     2    10    50
#> 3     2     0    12

Created on 2020-03-28 by the reprex package (v0.3.0.9001)