Question 1 has already been solved in this section. Look for it.
Question 2:
450 * y = n^3
i.e 3^2 * 5^2 * 2 = n ^ 3
We know that y is an integer and n is also an integer.
Hence, y would be equal to 3 * 5 * 2^2 to make it a perfect cube.
Which is the answer you have.
Question 3:
Upstream: V-3 miles per hour
Downstream : V+3 miles per hour
Distance: 90 miles
Hence,
Time_Upstream = 90/(V-3)
Time_Downstream = 90/(V+3)
Now, it says that
Time_Upstream = 1/2 + Time_Downstream
i.e
90/(V-3) = 1/2 + 90/(V+3)
Sovling, you get V = 33.
Time_Downstream will then be equal to 90/(36) = 2.5