how to Save A random generated number and use it

I want to use a random generated number and save it in a variable. i tried using X = public float x = Random.Range(minNb, maxNb) but it doesn't work. Here is the rest of my script.

 using System.Collections;
 using System.Collections.Generic;
 using Unity.Mathematics;
 using UnityEngine;
 
 public class Score : MonoBehaviour
 {
 
     public float score = 0;
     public float scoreMulti = 5;
     public float minNb = 100;
     public float maxNb = 200;
     
 
     // Start is called before the first frame update
     void Start()
     {
         
     }
 
     // Update is called once per frame
     void Update()
     {
         score += Time.deltaTime * scoreMulti;
 
         if (score == 70)
         {
            //save random generated number
         }
     }
 }