7 4/5 - 18 6/15 = 7 4/5 - 18 2/5

= (5*7 +4)/5 - (5*18 +2)/5

= 39/15 - 92/5

= (39 -92)/5

= -53/5

= -10 3/5

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If you meant 18 6/15 - 7 4/5, the math is the same but the numbers are swapped and the ending sign is positive instead of negative.

= (5*7 +4)/5 - (5*18 +2)/5

= 39/15 - 92/5

= (39 -92)/5

= -53/5

= -10 3/5

_____

If you meant 18 6/15 - 7 4/5, the math is the same but the numbers are swapped and the ending sign is positive instead of negative.