I got thius right because I'd seen this error before, but also think more people would not have picked up the error if 600000 had been offered as an answer, i.e. 500000+'50000'+'50000'.

The other obvious wrong answer would be 500000, i.e. if you assume the truncation occurs to the right i.e. 500000 becomes '00000' (leading '5' dropped) and you then get 500000+'0'+'0'.

I'd be interested to see what logic is supposed to arrive at either 15000 or 150000 since I can't see any obvious way to get either of these.