Đáp án:)
a, ( x - 31 ) * 13 : 11 = 65
( x - 31 ) * 13 = 65 x 11
( x - 31 ) * 13 = 715
x - 31 = 715 : 13
x - 31 = 55
x = 55 + 31
x = 86
b, 87, 5 * x + 1.25 * x = 20
( 87,5 + 1,25 ) * x = 20
88,75 * x = 20
x = 88,75 : 20
x = 4,4375
c, ( x - 42 ) * 13 : 11 = 18
( x - 42 ) * 13 = 18 * 11
( x - 42 ) * 13 = 198
x - 42 = 198 : 13
x - 42 = 15, 23
x = 15,23 + 42
x = 744/13
d, ( x - 53 ) * 13 : 11 = 91
( x - 53 ) * 13 = 91 * 11
( x - 53 ) * 13 = 1001
x - 53 = 1001 : 13
x - 53 = 77
x = 77 + 53
x = 130
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