To subtract these fractions we need a common denominator and a form of one for the conversion. As often with fractions of this nature just multiply the denominators. 4 * 15 = 60.
4*9/4*15 = 36/60
15*1/15*4 = 15/60
36/60 - 15/60
= 21/60 = 7/20
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9/15 -1/4=?