Using the concept of division works really well in such questions. It is good to remember that division is nothing but grouping. So when I say that a number is divided by 342, it means I split that number into groups of 342 and 47 was leftover.
342 is divisible by 19 so when you divide the same number by 19, every group of 342 will be cleanly split in groups of 19 and nothing will be left over. (This is what happens in most such cases.)
We only need to worry about the leftover 47. You can split 47 in two groups of 19 and then 9 will be leftover. Hence your remainder here will be 9.
(Note: For further clarity, refer to the theory of division.)
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