Respuesta :
a + c = 490....a = 490 - c
3a + c = 990
3(490 - c) + c = 990
1470 - 3c + c = 990
-3c + c = 990 - 1470
-2c = - 480
c = -480 / -2
c = 240 <=== there were 240 children
a + c = 490
a + 240 = 490
a = 490 - 240
a = 250 <== there were 250 adults
3a + c = 990
3(490 - c) + c = 990
1470 - 3c + c = 990
-3c + c = 990 - 1470
-2c = - 480
c = -480 / -2
c = 240 <=== there were 240 children
a + c = 490
a + 240 = 490
a = 490 - 240
a = 250 <== there were 250 adults