My problem in (ii) find the mass of the particle. My answer comes as 0.54 kg whereas the answer is given as 1.98 kg.
My working: EPE gained=(1.5*2-(2.4))*.5*90=27
KE=.5*m*10^2
thus m=.54
Mark scheme:

I think the mark scheme is wrong as the wire was prestreched by (2.4-1.6)m as shown in the diagram. But mark schemes are rarely wrong. Can you point out the fault in my reasoning? Thanks for reading.

2. initial stretch ... $\displaystyle 2.4-1.6 = .8$ m

final stretch ... $\displaystyle 3 - 1.6 = 1.4$ m

$\displaystyle \Delta U = -\Delta KE$

$\displaystyle \frac{150}{2}(1.4^2 - .8^2) = \frac{1}{2}m(10^2 - 0^2)$

$\displaystyle 150(1.4^2 - .8^2) = 100m$

$\displaystyle m = 1.5(1.4^2 - .8^2) = 1.98$ kg

3. Originally Posted by skeeter
initial stretch ... $\displaystyle 2.4-1.6 = .8$ m

final stretch ... $\displaystyle 3 - 1.6 = 1.4$ m

$\displaystyle \Delta U = -\Delta KE$

$\displaystyle \frac{150}{2}(1.4^2 - .8^2) = \frac{1}{2}m(10^2 - 0^2)$

$\displaystyle 150(1.4^2 - .8^2) = 100m$

$\displaystyle m = 1.5(1.4^2 - .8^2) = 1.98$ kg
Thanks, rather silly mistake I see me do.