Răspuns :
l----------l---------l--------l---------l--------l 30 elevi
-----------
atletism
l--------l--------l--------l--------l rest
---------
role
l--------l--------l--------l rest
---------
sarituri
l--------l--------l rest
-----------------
nu participa la concurs
30:5=6 atletism
30-6=24 rest
24:4=6 role
24-6=18 rest
18:3=6 sarituri
6+6+6=18 in total
30-18=12 nu participa
-----------
atletism
l--------l--------l--------l--------l rest
---------
role
l--------l--------l--------l rest
---------
sarituri
l--------l--------l rest
-----------------
nu participa la concurs
30:5=6 atletism
30-6=24 rest
24:4=6 role
24-6=18 rest
18:3=6 sarituri
6+6+6=18 in total
30-18=12 nu participa