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Which of the following ratios is not equivalent to $21:27$?

If $a:b=2\frac{1}{2}:1\frac{2}{3}$ and $b:c=1\frac{1}{4}:3\frac{1}{2},$ then find $a:b:c$.

If $1:3::x:18$ then find the value of $x$

If $a:b=3:5$ and $a:c=5:7$, then $\left(b-c\right):\left(b+c\right)$ equals to

If $25, z, z,49$ are in proportion, then find the value of $\mathrm{z}$.

Which of the following are not in proportion?

Fill in the blanks

$\frac{15}{18}=\frac{10}{\square }$

$1500$ sheets are required to make $100$ notebooks. How many sheets will be required to make $12$ notebooks