Tính bằng cách thuận tiện nhất. a) 154 680 × 15 + 268 000 × 15 + 77 320 × 15

a) 154 680 × 15 + 268 000 × 15 + 77 320 × 15

= 15 × (154 680 + 268 000 + 77 320)

= 15 × 500 000

= 7 500 000

b) \[\frac{5}{7}\,\, \times \,\,\frac{1}{{10}}\,\, + \,\,\frac{2}{5}\,\, \times \,\,\frac{5}{7}\,\, - \,\,\frac{1}{2}\,\, \times \,\,\frac{5}{7}\]

= \[\frac{5}{7}\,\, \times \left( {\frac{1}{{10}} + \frac{2}{5} - \frac{1}{2}} \right)\]

= \[\frac{5}{7}\,\, \times \left( {\frac{1}{{10}} + \frac{4}{{10}} - \frac{5}{{10}}} \right)\]

= \[\frac{5}{7}\,\, \times 0\] = 0