Which of the following numbers will replace the question mark (?) in the given series?136, 204, 306, ?, 688.5

Let's check the scaling factor between consecutive numbers in the series:$\frac{204}{136}=1.5$$\frac{306}{204}=1.5$ The terms increase by a constant multiplier of $1.5$. Let's use this factor to compute the next term:
$$\text{Next term}=306\times 1.5=459$$
Let's verify with the last term of the series:
$$459\times 1.5=688.5$$
This matches the sequence perfectly. The missing number is 459.