Can you explain 7 question of mixture alligation from practice section , I didn't understand the explanation provided there

so the best method to solve this question would be alligation-cross method.
let original stations = x
new stations = 6mm more than original = x+6
their mixture gives 1.5mm more than original = x+1.5
so putting these values in the cross,

x x+6
\ /
x+1.5
/ \
a b

=> a (x+6)-(x+1.5) x + 6 - x - 1.5 6 - 1.5 4.5 3
_ = _________ = __________ = ______ = ____ = __ = 3:1
b (x+1.5)-(x) x + 1.5 - x 1.5 1.5 1

hence final ratio of original:new = 3:1

hope this helps :)