A90 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.

1. 0.3 •4 is the same as 4 groups of 0.32. 7•0.8 is the same as 7 groups of 0.83. it increases by 0.5 every time, so 0.5x4 it increases by 0.9 every time, so 0.9x5. 0.9•12=10.86. 0.47•5=2.35i can finish the next ones in a separate post