a string 30m stretch to 35m, by what would be the length when stretched by 400N, assuming the extension limit is not exceeded

The problem seems to be missing some information. Normally, to solve this problem, we would need to know the force that was applied to stretch the string from 30m to 35m. This is because the extension of a string under a force follows Hooke's law, which states that the extension of a spring (or in this case, a string) is directly proportional to the force applied, provided the elastic limit is not exceeded.

The formula for Hooke's law is F = kx, where F is the force applied, k is the spring constant, and x is the extension of the string.

If we knew the force that caused the string to stretch from 30m to 35m, we could calculate the spring constant (k) and then use that to find out the extension when a force of 400N is applied.

Without that information, we can't provide a specific answer to the question.