a) 127 - ( $2^{3}$ . 7 + $3^{4}$ : 3 )
= 127 - ( 8 . 7 + $3^{3}$ )
= 127 - ( 56 + 27 )
= 127 - 83
= 44
b) $5^{20}$ : ( $5^{15}$ . 6 + $5^{15}$ . 19 )
= $5^{20}$ : [ $5^{15}$ ( 6 + 19 ) ]
= $5^{20}$ : [ $5^{15}$ . 25 ]
= $5^{20}$ : [ $5^{15}$ . $5^{2}$ ]
= $5^{20}$ : $5^{17}$
= $5^{3}$