First write down the unit ratio that is involved.
In this case, it's train carriage/doors.
Next, we set up our proportion.
We know that for every train carriage, there are 4 doors.
So we have 1/4.
We are asked how many doors will there be is there are 7 carriages.
So we have 7/x.
The 7 must go on the top because our unit ratio says
that we must put train carriage/doors.
So we have 1/4 = 7/x.
Solving from here, we use the means-extremes property.
This states that the product of the extremes, (1)(x),
equals the product of the means, (4)(7).
So we have 1x = 28 or just x = 28.
This means that for 7 carriages, there are 28 doors.