Din cantitatea de 30l de lapte 1 pe 5 este folosita pentru budinca,1 pe 3 pentru cacao cu lapte, iar restul pentru desertul de la ora 17: 00. cati litri de lapte s-au folosit la desert?

This is the concept of algebra, we are required to find the amount of milk used for dessert given that we initially had 30 l of milk, given that 1/3 was used for pudding, 1/5 was used for cocoa and the rest for dessert. 
The initial amount of milk is represented by fraction 1/1. The remaining amount of milk will be given by:
[initial fraction of milk]-[total fraction of milk used]
fraction of milk used was:
1/5+1/3=8/15
Therefore the remaining fraction was:
1/1-8/15
=7/15
Thus the amount that was used for dessert was:
7/15of 30
=14 l.
The answer is 14 l