c= cartea
[tex] \frac{1}{5} [/tex] x c +4 = [tex] \frac{1}{4} c[/tex]
[tex] \frac{1c}{5} +4 = \frac{1c}{4} [/tex]
[tex] \frac{4c}{20} +4= \frac{5c}{4} [/tex] | - [tex] \frac{4c}{20} [/tex] ( am amplificat prima fractie cu 4 si a doua cu 5, pt ca la numitor sa am acelasi nr, adica 20)
[tex] \frac{c}{20} [/tex] = 4 => c= 20 x 4= 80 pagini
