differential equation class radioactive decay assignment help

In the radioactive decay series of uranium 238U, an isotope of lead β-decays with a half-life of 26.8 92

minutes into bismuth, which in turn β-decays with a half-life 19.9 minutes into polonium: 

Its is my first time see this kind of chemistry math question, could someone solve this for me? I ask all my friends no one can do this 

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