u r dealing with proportions...
x/8 = 3/4
cross multiply...multiply denominator of one, by numerator of the other and vice-versa
(4)(x) = (3)(8)
4x = 24
x = 24/4
x = 6
sub it back into the proportion
6/8 = 3/4.....notice that if u reduce 6/8, it equals 3/4. Proportions are nothing but equivalent fractions.
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2/5 = x/40
cross multiply
(5)(x) = (40)(2)
5x = 80
x = 80/5
x = 16
check..
2/5 = 16/40... u can aslo cross multiply to check
(2)(40) = (5)(16)
80 = 80...correct
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1/8 = x/12
cross multiply
(8)(x) = (1)(12)
8x = 12
x = 12/8
x = 3/2
check...
1/8 = (3/2) / 12
cross multiply
(8)(3/2) = (12)(1)
24/2 = 12
12 = 12 (correct)
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I am gonna leave u with the last one....u can do this :)