How can I solve the simple ratio in this exercise?

Question

I am studying proportions on my own, but how can I solve this exercise?

In a kindergarten class, the value of the ratio between boys and girls is $3.5$. If the number of children is $140$, then how many girls are there?

What is the property or formula to solve this?

Answer 1

If there are $g$ girls there are $3.5g$ boys, so $$g+3.5g=140$$ Unfortunately, you have some fractional kids here.

Answer 2

I took "value of the ratio" is $3.5$ to mean that $\frac {\text {#number of boys}}{\text {#number of girls}} = 3.5$.

So if $g$ = number of girls and $b$ = number of boys we have:

$\frac bg = 3.5$ and $b + g = 140$.

$b = 3.5g$ so $3.5g + g = 140$

$4.5g = 140$

$9g = 280$

$g = \frac {280}9 = 31 \frac 19$ and $b = 108\frac 89$.

I'm assuming that the ratio being $3.5$ was a rounding estimation and that there are $31$ girls and $108$ boys.

So the actual ratio is $\frac {108}{31} \approx 3.5$.