Fractions represent the manner of writing parts of whole numbers (integers). Rules for operations with fractions differ from rules for operations with integers. Students face difficulties in understanding fractions, especially operations with fractions. These difficulties are well known in didactics of Mathematics throughout the world and there is a lot of research regarding problems in learning about fractions. Methods for facilitating understanding fractions have been discovered, which are essentially related to visualizing operations with fractions.

Coal combustion power plant Kolubara A has five thermal units with total installed capacity of 270 MW operates as a part of Public Enterprise which has planned to accept and apply the Clean Development Mechanism (new power plants on lignite and natural gas with higher energy efficiency as well as activities related to the rehabilitation and modernization of existing facilities). Two units were put in operation before more than fifty years and in order to extend the lifetime of these blocks in the process of revitalization, in this paper, beside an existing option which considers coal (lignite) combustion, are analysed possible options of thermal power units with gas and renewable energy sources: biomass, renewable solid fuels, waste materials, wind and sun. Paper presents seven energy system options of the thermal unit status which are described with suitable chosen indicators in respect to different aspects of sustainability: economical, ecological, social and resources indicators. In this paper energy indicators are defined as a tool for the sustainability assessment of energy system options. ASPID method of multi-criteria assessment was used to calculate general index of sustainability and obtained ranking of options. This study presented and analysed cases, with predefined constraints when priority has environmental indicator and different conditions where one of sub-indicators have priority, as an example of applying multi-criteria method.

The focus of this study was on genetic diversity of TB horse population raised in B&H. Genomic DNA was genotyped by using 17 microsatellite markers. A total of 103 alleles were detected. The average number of alleles per locus was 6.059 and effective number of alleles was 3.293. Means of observed and expected heterozygosity were calculated 0.645 and 0.696, respectively. The average PIC values was 0.649 and inbreeding coefficient was 0.090. Based on all observed parameters, ASB2 locus showed the highest genetic diversity while locus HMS2 was the least diverse. These results suggest that the population of TB horses from B&H is not affected by substantial loss of genetic diversity, indicating the presence of reasonably high level of genetic variability.