a/b=2/5 --> schimbam mezii intre ei => a/2=b/5=k a/2=k=> a=2k b/5=k=> b=5k inlocum: 7×2k-2×5k/4×2k+3×5k= 14k-10k/8k+15k= 4k/23k se simplifica k cu k si raspunsul final este 4/23