Question

Si a/b = 3/5 ; b/c = 5/8 y a+c = 33; calcula el valor de "a"

Answer 1

Pongamos constantes para igualar las fracciones con denominadores y numeradores iguales, así:
a = 3k y b = 5k y c = 8k
entonces 
a + c = 33
3k + 8k = 33
11k = 33
k = 3
reemplazando en "a": a = 3k
a = 3(3)
a = 9

Answer 2

Ecuaciones
a/b = 3/5
b/c = 5/8
a+c = 33

Solución
a+c=33
a = 33-c

b/c = 5/8
b = 5c/8

a/b = 3/5
(33-c)/(5c/8) = 3/5
[8(33-c)]/5c = 3/5
(264-8c)/5c = 3/5
264-8c = 3(5c)/5
264-8c = 15c/5
264-8c = 3c
-8c-3c = -264
-11c = -264
c = 264/11
c = 24

a+c=33
a+24=33
a=33-24
a = 9

b= 5c/8
b=5(24/8)
b=15
El valor de a es 9

Saludos...