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#1
12/5=20/x
12x=5*20
12x=100:12
x=8 1/3
#2
2/5/x=1/5/2/3
2/5*2/3=1/5x
-1/5x=-4/15:(-1/5)
x=1 1/3
#3
3.75/1/8=2 1/3/x
3,75x=1/8*7/3
3 3/4x=7/24:3 3/4
x=7/24*4/15=7/90
#4
1,2/x=3/7
1,2*7=3x
-3x=-8,4:(-3)
x=2,8
#6
1/0,5=90/x
x=90*0,5
x=45
#7
x/5=2,4/20
20x=2,4*5
20x=12:20
x=0,6
#8
x/7/18=2/7/7/9
7/9x=7/8*2/7
7/9x=1/4:7/9
x=9/28
#9
27/x=3/4
27*4=3x
-3x=-108:(-3)
x=36
#10
45/5x=9/4
45*4=5x*9
-45x=-180:(-45)
x=4
#11
17/51=x/6
17*6=51x
-51x=-102:(-51)
x=2
#12
3 3/4/1 1/8=2 1/3/p
3 3/4p=1 1/8*2 1/3
15/4p=9/8*7/3
15/4p=21/8:15/4
p=21/8*4/15=0,7
#13
3/8=6/16
8+6=14
2/7/3=4/6/14
14+2/7=14 2/7
#5
1,5/0,3=9/3
1,5*3=0,3*9
4,5≠2,7