20)x+y=380
x-y =3·y
x=3y +y
x=4y
x+y=380
4y+y=380
5y=380
y=76
76+x=380
x=380-76
x=304
21)136:2=68 este 1/3
68·3=204 ⇒este numarul intreg
22)135:5=27
27·3=81 radu
27·2=54 alin
verificare (81:3):2=54:2=27
23)91:7=13 cast.ciprian
12·2=26 cast.andrei
4·13=52 cast.maria
verificare 13+26+52=91
24)14:2=7 portoc. a II a cutie
7+3=10 potoc. a III a cutie
7·3=21 portoc. in prima cutie