Question

An element with a mass of 800 g to Cave by 28% per minute. To the nearest minute how long will it take until they were 140 g of the element remaining?

Answer 1

I guess “to Cave” means decay.
After n minutes the amount remaining = original mass x (1 - 0.28)^n
800 x 0.72^n = 140
0.72^n = 140/800 = 0.175
n = log(0.175)/log(0.72) = 5.305
To the nearest minute it takes 5 mins until there is only 140 g remaining.

The weight remaining follows this sequence: 800, 576, 414.7, 298.6, 215, 154.8, 111.5
so the closest weight to 140 is after 5 minutes