You are correct. -30 is the greater number.
Let's look at the real number line.
-3 -2 -1 0 1 2 3
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If you look at the positive numbers, the higher numbers (the ones that are greater) are to the right of the lower numbers.
The same rule holds for negative numbers too. On the real number line, numbers on the right are greater than the numbers on the left.
If you extended the above real number line to include the numbers -30 and -48, you would get something like this.
-50 -40 -30 -29 -19
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Now -48 would be somewhere between -50 and -40, but would still be to the left of -30.
i.e. -30 is to the right of -48, therefore it is greater.