you gotta follow it through backward:
1. how many moles of sodium thiosulfate reacted with it? n=cv
2. what are the ratios? it seems 2 moles of thiosulfate react with __ moles of I2, we know the number of moles of thiosulfate so we can work out number of I2 there reacted using the ratio
3. this is the amount of moles of I2 in 25cm^3 so we need to scale it up to the 250cm^3 volume, so there are __ moles of I2 in 250cm^3
4. look at the ratios from the 1st equation given, how many moles of copper are needed to produce 1 mole of I2? we know the number of moles of I2 (from the scaled up volume) so we can use the ratios to work out number of moles of Cu.
5. we have number of moles of copper so we can work out its mass using number of moles= mass/MR
6. we now know mass of copper and mass of the alloy is given so we can work out percentage mass in the alloy.