It's not very clear, it does rather jump ahead without showing working.
How much charge is passed, at the given current for the given time? How many faradays does this correspond to?
How much charge would be used if you were using the whole of the initial solution, rather than just the sample indicated?
Given the stated efficiency, how much charge would be actually used to deposit all the metals in the alloy sample?
(I assume you know how to convert coulombs to faradays and mAh.)