Not sure if it's ok to add something but I will lol.

For b, this is what I've worked out:

100m = 100 000mm, 2.5cm = 25mm

So:

1/100 000 + 1/S = 1/25 - Find S

1/S = 1/25 - 1/100 000

1/S = 4000/100 000 - 1/100 000

1/S = 3999/100 000

S = 1 / (3999/100 000)

S = 25.006mm

Sub into original equation:

1/100 000 + 1/25.006 = 0.04mm and original is 1/25 which is 0.04mm

Does this look right?