Răspuns:
1. a-b=20 => a=b+20
2a/5=3b/4-13 aducem la acelasi numitor (20=5*4)
15b-8a=13*20 15b-8a=260
15b- 8(b+20)=260 15b-8b -160=260
7b=260+160 7b=420 b= 60
a= 60+20=80 a= 80
2. 5a/9 +3b/7=1605 5a/9- 3b/7=855 le scadem
(3b+3b)/7= 1605-855 = 750
6b/7=750 b= 750*7 : 6=125*7=875 b=875
5a/9=3*875:7 +855= 375 +855=1230
a= 1230*9/5=2214 a=2214