Find the missing number __ : 10 = 70 : 100.

Hint: The concept of ratio and proportion will be used in the question. We can easily convert any ratio into a fraction by keeping the first term as numerator and the second as denominator. Mathematically,
${\text{a}}:{\text{b}} = \dfrac{{\text{a}}}{{\text{b}}}$

Complete step-by-step answer:
We have to find the missing number in __ : 10 = 70 : 100. Let us assume the missing number to be x, that is-
$x : 10 = 70 : 100$
We can convert these ratios into fraction as-
$\dfrac{{\text{x}}}{{10}} = \dfrac{{70}}{{100}}$
${\text{x}} = \dfrac{{70 \times 10}}{{100}} = 7$
Hence, the missing number is 7. This is the required answer.

Note: These problems can be solved by simple multiplication and division, what we need to do is just have knowledge of the concept of ratios and proportions. Instead of using fractions, we can use decimal notations as well-
$x : 10 = 70 : 100$
$\dfrac{{\text{x}}}{{10}} = 0.70$
${\text{x}} = 7.0 = 7$