Explain why when a firefighter rescues a dog that has fallen through ice on a lake,

This can spread their weight.

Explanation:

The ladder has a much larger surface area than the firefighter. Hence, his weight is spread out much more than usual, decreasing the pressure on the ice, preventing the ice from breaking/cracking.

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Explanation:

To find the half-lifes of the isotope we need to use the following equation:

    (1)

where Nt: is the amount of the isotope that has not yet decayed after a time t, N₀: is the initial amount of the isotope, t: is the time and : is the half-lifes.

By solving equation (1) for t we have:

Having that:

Nt = 450

N₀ = 3150 + 450 = 3600,

The half-lifes of the isotope is:

Therefore, 3 half-lives of the isotope passed since the rock was formed.

I hope it helps you!