Combining values from mutiple sensors

Configuration
1

I ran into a confusing situation to-day and would be grateful for some explanation.
I want to display a value on a Big Number Card on a Lovelace view. This card type suits my needs, I realize that perhaps other cards type could be used and the result formatted to simulate the Big Number Card but in this instance it is my preferred choice. The value is a composition of the values obtained from two sensors plus a little text. My problem is that when I create the new sensor to include the details, I am getting an “unknown” result. Coincidentally I also am changing these values from kph to mps by multiplying by 0.277777778. I have also tried the following code both with an without the factor multiplication.
Would be grate if someone could put me right and explain why I get unknown instead of something like 1.2/2.5 m/s

Sensor 1: sensor.hp2551ae_pro_v1_9_9_wind_speed
Sensor 2: sensor.hp2551ae_pro_v1_9_9_wind_gust
Sensor 3: (the one I want to create) Combo Wind & Gust

Alt 1

- platform: template
  sensors:
    combo_wind_gust:
      friendly_name: "Combo Wind & Gust"
      value_template: >
        {{ (states('sensor.hp2551ae_pro_v1_9_9_wind_speed') | float * 0.277777778) | round(1) }}/{{ (states('sensor.hp2551ae_pro_v1_9_9_wind_gust') | float * 0.277777778) | round(1) }} m/s
      unit_of_measurement: "m/s"
      unique_id: ComboWind
      icon_template: "mdi:weather-windy"

Alt 2

- sensor:
    - name: "Combo Wind & Gust"
      unique_id: combo_wind_gust
      icon: mdi:weather-windy
      unit_of_measurement: "m/s"
      state: >
        {% set wind = (states('sensor.hp2551ae_pro_v1_9_9_wind_speed') | float(0) * 0.277777778) | round(1) %}
        {% set gust = (states('sensor.hp2551ae_pro_v1_9_9_wind_gust') | float(0) * 0.277777778) | round(1) %}
        {{ wind }}/{{ gust }} m/s
2

By including this option:

      unit_of_measurement: "m/s"

Home Assistant is led to believe the sensor’s state value will be a number.

However, in both examples, the values are not numbers. The first example is a string because m/s is hard-coded into the value. The second example’s value is also a string for the same reason and also because it use / to display two values (wind and gust).

Modify the template to produce nothing more than a number value.

3

Amazing, such a small component change and now the new sensor gives the expected values. Well spotted!