Hi there mastermin346

Well i'm reading this as the length of string is 90cm?

Therefore

90 = length of string #1 + length of string #2+ .... +length of string #5

And the difference in each length, shaped as a circle is, adding 1 to the next radius so

Factoring out and then dividing

Grouping like terms on the RHS

You should be able to use the same logic to finish the rest.