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**i_zz_y_ill** If any one has the time to look atthis that would be great. Im stuck on (iv)

Q A radioactive element X decays into he radioactive element Y which decays into the stable eement Z. Thedecay can be modelled by th differential equations:

(1) dx/dt = -0.1X

(2)dy/dt=0.1x-0.2y

(3)dz/dt=0.2Y

Where x.y and z are the masses (in milligrams) of X,Y AND Z respectively at time t seconds. When t=0, the mass of X is 25mg, and there is no or Z.

(i) Solve eqn (1) to find x in terms of t,Sketch graph of your solution(DONE x=25e^-0.1t)

(ii)se your solution for x to solve (2) to find y in terms of t. (DONE y=25(e^-0.1t-e^-0.2t))

(iii)Calculatethe range of times for which the amount of Y is increasng. Sketch a graph of the mass of Y against time, showing the maximum alue. (DONE T IS SMALLER THAT 10LN2)

(iv)*Without solving eqn (3), show that x+y+z is constant. Hence find z in terms of t, and verify this satidfies eqn(3) and the initial conditions.

for (iv) the mark scheme does dx/dt+dy/dt+dz/dt=0 which implies a constant which I sortov understand but then in the mark scheme it requires a mar to get the constant as 25 i.e x+y+z=25 and I have no idea how to get this or find z in terms of t, the very last bit verifying should be easy. Does any one know how? thanks