3/5=12/x.
Cross multiply: 3x=60, x=20 or
12 is 4 times 3, so x is 4 times 5=20
16/24=x/12.
12 is half 24 so x is half 16=8
12/x=3/4.
3 is a quarter of 12, 4 is a quarter of 16, so x=16
x/8=25/4.
8 is twice 4 so x is twice 25=50
x/42=1/6.
6 times 7 is 42, 1 times 7 is 7 so x=7
12/x=2/9.
6 times 2 is 12, so 6 times 9 is 54, so x=54