exam Maths Puzzle Numerical Ability

Hello There,

I seem to be stuck on theses problems. Any help would be greatly appreciated. Thank you. Here are the problems.

Find the domain of the following:

f(t)= 4.5

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exam Other Question

If there are 6 * 10^9 people on the Earth and there is enough freshwater
to provide each person with 8.79 * 10^5 L, how much freshwater is on the
Earth? 8x+1+4x=3

my answer:
8x+4x=1+3

12x = 4 *(12x over 4/4 over 4)
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4 4 x=3 (answer)