Concept:
Whenever a ratio of two quantities is given, we may assume the quantities to be proportional to the terms of the ratio.
Given:
\[
x:y=4:5
\]
Let
\[
x=4k,\qquad y=5k
\]
where \(k\) is a non-zero constant.
Step 1: Substitute the values into the first expression.
\[
5x+4y
\]
\[
=5(4k)+4(5k)
\]
\[
=20k+20k
\]
\[
=40k
\]
Step 2: Substitute the values into the second expression.
\[
10x-2y
\]
\[
=10(4k)-2(5k)
\]
\[
=40k-10k
\]
\[
=30k
\]
Step 3: Form the required ratio.
\[
(5x+4y):(10x-2y)
=
40k:30k
\]
Cancelling the common factor \(10k\),
\[
=4:3
\]
Therefore,
\[
\boxed{4:3}
\]