Hello!
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I think that 12/42=10/35 is a true proportion. 10/9=11/10 are almost a true proportion, but 10/9 is 1.1111111111..., and 11/10 is 1.1. 48/56=40/48 and 1/2=3/4 are different numbers when in decimal form.
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I hope this helps and have a great day! :)