I tutor Math here in Canada, and it is a bit advanced for 12 years old here. Anyhow, I attach my solution. Please pardon my terrible handwriting.
Also, I don't guarantee my calculations to be free of arithmetic errors! I did check, but these types of mistakes can happen. This problem would be even more challenging, but they chose the numbers with care.
We can use 104 -64 = 40 coins
Helen has 40 more coins in $0.50 than Ivan in $0.50
$0.5-$0.2 = $0.3
40 x $0.3 = $12
Helen has $12 more than Ivan
40 x 2.7 = 108g = 0.108kg
1.134 - 0.108 = 1.026kg
I must have been very tired! First, I made the error of 12 cents rather than 12 DOLLARS. Then, I multipled the number of 50 cent coins by the mass of the 20 cent coin instead of by the mass of the 50 cent coin. Big oopsies! Sorry. Also, your method was much simpler. Thank you for being gracious in your response.
It's a English test question in a Maths paper.