Mathf.InverseLerp

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public static float InverseLerp (float a, float b, float value);

Description

Calculates the linear parameter t that produces the interpolant value within the range [a, b].

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour { public float walkSpeed = 5.0F; public float runSpeed = 10.0F; public float speed = 8.0F; void Start() { float parameter = Mathf.InverseLerp(walkSpeed, runSpeed, speed); } }

See Also: Lerp.