Pre Algebra: Proportions Practice
1. If $\dfrac{x}{16}=\dfrac{12}{y}$, then what is the value of $xy$?
ANSWER: (C) $192$
Cross multiply. Through cross multiplying you will get xy=192. (x ⋅ y = xy and 16 ⋅ 12= 192)
2. If $\dfrac{a}{8}=\dfrac{1}{16}$, then what is the value of $a$?
ANSWER: (A) $\dfrac{1}{2}$
Cross multiply. 16 ⋅ a = 16a and 8⋅ 1= 8. After that you are left with 16a=8. We want to get "a" alone so we must divide by 16. 8/16=1/2.
3. If $\dfrac{12}{n-2}=\dfrac{8}{2n}$, then what is the value of $n$?
ANSWER: (A) $-1$
Cross multiply. 12 ⋅ 2n and 8 ⋅ (n-2). That will give you 24n=8(n-2). Next distribute the 8. Then you will have 24n=8n-16. We want the n's to be together, so we subtract 8n from both sides. 24n-8n=16n. Now we have 16n=-16. To get n alone, we must divide both sides by 16. -16/16=-1.
4. The ratio of 1.8 to 3 is not equal to which of the following ratios?
ANSWER: (B) 3 to 10
Process of elimination! We know that 1.8 to 3 IS equal to 18 to 30, since that is what you'd get if you multiplied both numbers by ten. We know 1.8/3 IS equal to 6/10 since 1.8/3=3/5 and 3/5=6/10. 1.8/3 is also equal to 9/15 since that's what you'd get if you multiplied 1.8/3 by 5/5. 1.8/3 IS also equal to .6/1 since that's what you'd get if you divided the bottom and the top by 3. (1.8 ÷ 3=.6 and 3 ÷3= 1). The only option left is B.