20. 4p + 15c = 53
4p + 10c = 38, le scadem
___________________
/ 5c = 15
c = 15 : 5
c = 3 lei
4p + 10 * 3 = 38
4p = 38 - 30
4p = 8
p = 2 lei
19.
9 * 5 - 1 * 3 = 45 - 3 = 42 puncte
8 * 5 - 2 * 3 = 40 - 6 = 34 puncte Andrei a rezolvat 8 probleme si 2 nu a rezolvat
21.
a + b = 455
a/b = 3/4 ⇒ a = 3b/4
3b/4 + b = 455
3b + 4b = 1820
7b = 1820
b = 1820 : 7
b = 260
a = 455 - 260
a = 195