Coroana. Va rog frumos.
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Explicație pas cu pas:
17.
Numere de doua cifre sunt 10, 11,......, 98, 99, adica 90 de numere
a) 19, 29, 39, 49, 59, 69, 79, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 = 18 numere
(18/90)*100 = 20%
b) 12, 15, 18, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, 63, 66, 69, 72, 75, 78, 81, 84, 87, 90, 93, 96, 99 = 29 numere
(29/90)*100 = 32,(2)%
c) 10, 11, 12, 13, ...., 23, 24 = 15 numere
(15/90)*100 = 16,(6)%
d) 15, 25, 35, 45, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 65, 75, 85, 95 = 18 numere
(18/90)*100 = 20%
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18.
a) 5a/100 = 8b/100
5a = 8b
a/b = 8/5
b) 12a/100 = 20b/100
12a = 20b
a/b = 20/12 = 5/3
c) 7a/100 = 11b/100
7a = 11b
a/b = 11/7
d) 15a/100 = 25b/100
15a = 25b
a/b = 25/15 = 5/3
19.
40a/100 = b
40*b/100 = (40*40a/100)/100 = 16a/100 = 16% din a
20.
A = 20x/100
B = 30x/100
A/B = (20x/100) : (30x/100) = 20x/100*100/30x = 2/3