Which part of "what's your intuitive answer" did you misunderstand?

Notwithstanding that, the simpler form of the equation recognises that 'a' and 's' are stated to be the same, ergo:

v1^2 - u1^2 = v2^2 - u2^2

And, since u1 is stated to be zero:

u2^2 = v2^2 - v1^2

So u2 = root (v2^2 - v1^2)

i.e. u2 = root (v2^2 - 900)

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So, taking the answer for 40mph, which is that a child who would be missed by a car originally doing 30mph would be hit at 26.5mph by a car originally doing 40mph, does it strike you as being quite important to drive slowly in built-up areas?

After all, if you managed a sustained speed of 40mph through a 3-mile stretch of urban roads, you stand to gain merely 90 seconds. Is that a sufficient 'reward' for taking the risk of maiming or killing a child?