5 $к{м}^2$ = (5 * 100) га = 500 га;
30 $к{м}^2$ = (30 * 100) га = 3000 га;
2300 а = (2300 : 100) га = 23 га;
68000 а = (68000 : 100) га = 680 га.

4 га = (4 * 10000) ${м}^2$ = 40000 ${м}^2$;
50 а = (50 * 100) ${м}^2$ = 5000 ${м}^2$;
10 а 30 ${м}^2$ = (10 * 100) ${м}^2$ + 30 ${м}^2$ = 1000 ${м}^2$ + 30 ${м}^2$ = 1030 ${м}^2$.