type 1 basic concepts of ratio & proportion Practice Questions Answers Test with Solutions & More Shortcuts

Question : 11 [SSC CPO S.I.2006]

If x : y = 2 : 3, then the value of ${3x+ 2y}/{9x+ 5y}$ is equal to

a) $1/2$

b) $5/14$

c) $4/11$

d) $11/4$

Answer: (c)

Given, $x/y = 2/3$ ... (i)

Expression = ${3x + 2y}/{9x + 5y}$

= ${3x/y + 2}/{9x/y+ 5} = {3×2/3+2}/{9×2/3+5}$[from (i)]

= ${2 + 2}/11 =4/11$

Question : 12 [SSC CPO S.I.2006]

The mean proportional between $(3 +√2)$ and $(12 - √32)$ is

a) 6

b) ${15 -3√2}/2$

c) $2√7$

d) $√7$

Answer: (c)

Using Rule 14
Mean Proportion - Let x be the mean proportion between a and b, then a:x::x:b (Real condition)
$a/x = x/b ⇒ x^2 =ab$
$x =√{ab}$
So, mean proportion of a and b = $√{ab}$
If x be the mean proportion between (x - a) and (x - b) then what will be the value of x ?
$x = {ab}/{a+b}$

Mean proportional

=$√{(3+√2)(12-√{32})}$

= $√{(3+√2)4(3-√2)}$

= 2$√{9 - 2} = 2√7$

Question : 13 [SSC SO 2006]

If a, b, c are three numbers such that a : b = 3 : 4 and b : c = 8 : 9, then a : c is equal to

a) 3 : 2

b) 1 : 2

c) 2 : 3

d) 1 : 3

Answer: (c)

We can write a : c by compounding a : b and b : c

$a/c = a/b × b/c, a/c = 3/4 × 8/9, a/c =2/3$

a : c = 2 : 3

Using Rule 18
If A:B = x:y and B:C = p:q then
(i) A:C = xp : yq
(ii) A:B:C = (x:y) × p:qy =xp:yp:qy
It is done as follows:
A:B = x:y
B:C = p:q
A:B:C = xp:yp:qy

A : C = xp : yq

= 3 × 8 : 4 × 9 = 2 : 3

Question : 14 [SSC CPO S.I.2006]

If a : (b+c) = 1 : 3 and c : (a+ b) = 5:7, then b : (a+c) is equal to

a) 1 : 3

b) 2 : 1

c) 2 : 3

d) 1 : 2

Answer: (d)

a : (b+c) = 1: 3

${b+ c}/a = 3/1 ⇒ {b+c}/a+1 =3/1$+1

${a+ b + c}/a= {3+1}/1 = 4/1$ ... (i)

Similarly, ${a + b}/c = 7/5$

${a + b + c}/c = 12/5$ ... (ii)

On dividing (i) by (ii),

$c/a = {4 × 5}/12 = 5/3$ =k ... (iii)

From equation (i), b = 4k

$b/{a + c} = {4k}/{3k + 5k} =1 : 2$

Question : 15 [SSC CGL Prelim 2005]

If a : b = b : c, then $a^4 : b^4$ is equal to

a) $c^2 : a^2$

b) $b^2$ : ac

c) $a^2 : c^2$

d) ac : $b^2$

Answer: (c)

$a/b = b/c$

$b^2 = ac ⇒ b^4 = a^2c^2$

$a^4/b^4 = a^4/{a^2c^2} = a^2/c^2$

IMPORTANT quantitative aptitude EXERCISES

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