a, 135 + 67 - 181 = 202 - 181 = 21
b, 203 - ( 176 - 94 ) = 203 - 82 = 121
c, 58 × 3 - ( 215 - 70 ) : 5 = 58 × 3 - 145 : 5 = 174 - 29 = 145
d, 31 × 56 + 69 × 72 + 31 ×44 + 69 × 28
= 31 × ( 56 + 44 ) + 69 × ( 72 + 28 )
= 31 × 100 + 69 × 100
= 100 × ( 31 + 69 )
= 100 × 100
= 10000
e, $\frac{5}{7}$ - 25% = $\frac{13}{28}$
g, 1$\frac{1}{2}$ - ( $\frac{3}{4}$ - 20% )
= 1$\frac{1}{2}$ - $\frac{11}{20}$
= $\frac{3}{2}$ - $\frac{11}{20}$
= $\frac{19}{20}$
h, $\frac{17}{19}$ × $\frac{5}{23}$ + $\frac{17}{23}$ × $\frac{8}{19}$ + $\frac{10}{23}$ × $\frac{17}{19}$
= $\frac{17}{19}$ × ( $\frac{5}{23}$ + $\frac{10}{23}$ ) + $\frac{17}{23}$ × $\frac{8}{19}$
= $\frac{17}{19}$ × $\frac{15}{23}$ + $\frac{17}{23}$ × $\frac{8}{19}$
= $\frac{255}{437}$ + $\frac{136}{437}$
= $\frac{17}{19}$
i, bó tay :))
