Tính bằng cách thuận tiện:  a) 5/3 x 13/15 x12/5 x 15/13

a) \[\frac{5}{3} \times \frac{{13}}{{15}} \times \frac{{12}}{5} \times \frac{{15}}{{13}}\]

\[ = \frac{{5 \times 13 \times 12 \times 15}}{{3 \times 15 \times 5 \times 13}}\]

\[ = \frac{{12}}{3}\]

= 4

b) \[\frac{{23}}{{70}} \times 575 - \frac{{23}}{{70}} \times 505\]

\[ = \frac{{23}}{{70}} \times \left( {575 - 505} \right)\]

\[ = \frac{{23}}{{70}} \times 70\]

= 23