[tex] \bf \cfrac{a}{b}\div \cfrac{c}{d}\implies \cfrac{a}{b}\cdot \cfrac{ d}{c}\qquad therefore \qquad \cfrac{3}{5}\div \cfrac{3}{15}\implies \cfrac{3}{5}\cdot \cfrac{15}{3}\implies \cfrac{3}{3}\cdot \cfrac{15}{5}\implies 3 [/tex]
The rule here is that one inverts (finds the reciprocal of) the divisor (3/15) and then multiplies:
In 3/5÷3/15
we arrive at 3/5 * 15/3 = 3.
