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25.
x : y = 1 : 3
y : z = 5 : 7
z : k = 9 : 11

to find the ratio between x, y, z, we need LCM(3,5) = 15 <---- for y
so x : y : z = 5 : 15 : 21

to find the ratio between y, z, k, we need LCM(7,9) = 63  <---- for z
so y : z : k = 45 : 63 : 77

then the total ratio is
x : y : z : k = 15 : 45 : 63 : 77

hence, the value of x : k = 15 : 77


..hope this is helpful