a:b=c:d=3:5=?

To find the value of `a:b=c:d=3:5`, we need to use the concept of ratios.

A ratio compares the relative sizes of two or more quantities. In this case, we are given that `a:b` and `c:d` are both equal to `3:5`.

To find the common ratio between `a:b` and `c:d`, we can divide the first number in each ratio by the second number:

For `a:b`, we have `a/b = 3/5 = 0.6`.
Similarly, for `c:d`, we have `c/d = 3/5 = 0.6`.
Both ratios have a common ratio of `0.6`, which means that `a:b=c:d=0.6`.

Therefore, `a:b=c:d=3:5=0.6`.