18.
( 24 . 5 - $5^{2}$ . 2 ) : ( 5 . 2 ) - 3 = ( 24 . 5 - 25 . 2 ) : 10 - 3
= ( 120 - 50 ) : 10 - 3
= 70 : 10 - 3
= 7 - 3
= 4
19.
[( $5^{2}$ . $2^{3}$ - $7^{2}$ . 2 ) : 2 ] . 6 - 7 . $2^{5}$ = [( 25 . 8 - 49 . 2 ) : 2 ] . 6 - 7 . 32
= [( 200 - 98 ) : 2 ] . 6 - 7 . 32
= [ 102 : 2 ] . 6 - 7 . 32
= 51 . 6 - 7 . 32
= 306 - 224
= 82
20.
( 6 . $5^{2}$ - 13 . 7 ) . 2 - $2^{3}$( 7+ 3 ) = ( 6 . 25 - 13 . 7 ) . 2 - 8( 7+ 3 )
= ( 150 - 91 ) . 2 - 8( 7 + 3 )
= 59 . 2 - 8 . 10
= 118 - 80
= 38
#nocopy
