The rate at which a certain radioactive isotope decays is given by = negative 0.0032 y, where y is the amount of the isotope after t days. After approximately how many days would the amount of this isotope in a sample reach 75% of its original amount?

90 days.

Step-by-step explanation:

dy/dt = - 0.0032y

dy = -0.0032y dt

dy/y = -0.0032 dt

Integrating:

ln y = -0.0032t + c     where c is some constant

y = e^(-0032t) + e^c

y = A e^(-0032t)   where A is a constant.

At time 0 we can let y = 1 so in that case A = 1

When there is 0.75 of y left:

0.75 = e^-0032t

ln 0,75 =  -0032 t

t = ln 0.75 / -0032

t = 90.

c

step-by-step explanation:

c

b) 1/3 * 1/5

step-by-step explanation:

each smallest part is 1/15 since the written one is 1/5 and it's divided by 3 equal parts. (1/5 )/3 = 1/15 = 1/3 * 1/5