A 90 g sample of plutonium-238 has a half-life of 88 years. How long will it take for this sample to decay to

40 g?

The equation to solving this is 90/40 = 88 / X

knowing this I multiply 40 and 88 and 90 and x. I got 3520 and 90x. Now I divide 3520 by 90 and I get 39 and 1/9. So the time for the sample to decay is:

39 1/9 years.