How many people attending the picnic were:
In their 40s? 9 (40-49 group)
80 or older? 3 (80-89 group)
older than 20? Not enough information (20 IS NOT INCLUDED IN THE COUNT but we dont know how many are 20 y.o in the 20-29 group)
at least 50? 6 + 10 + 5 + 3 = 24 (50-59, 60-69, 70-79 and 80-89 group, 50 being the lowest age)
at most 19? 8 + 7 = 15 ( 0-9 and 10-19 group, 19 is the highest age)
at least 35? Not enough information ( we dont have an idea how many are aged 30-34 in the 30-39 group)
under 30? 8 + 7 + 15 = 30 ( 30 is not included, 0-9, 10-19 and 20-29 group)