Estimate the age of the galaxy?

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The galaxy’s age can be estimated using the radioactive decay of Uranium (produced in supernovae). The initial abundances of the 2 isotopes U235 and U238 are expected to be in the ratio U235/ U238 ? 1.65. The present abundance ratio is ? 0.0072. The decay rates of the isotopes are ?(U235) = 0.97 ×10^9 year-1 and ?(U238) = 0.15×10^9 year-1. Estimate the age of the galaxy.
I know the answer is 6.6×109 years, and that I should use the formula N(t)=N(o)e^-(decay constant)(time), but I cant figure out how to use the equation in this question, since there are 2 constants.
I would really appreciate some help with this question. Thanks.
 
The life time of a galaxy is about 6 billion years.
The earth is two-thirds of the way from the center and is about 4,5 billion years old
The milky way is in a state of accelerated collapse towards the galactic center.
This makes the galaxy about a billion years older than the earth.
The age of the galaxy is just over 5 billion years old with about 1 billion years left to exist.
 
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