The displacement of a body at any time t after starting is given by `s=15t-0.4t^2`. Find the time when the velocity of the body will be `7ms^-1`.

Explanation

To solve the problem step by step, we need to find the time when the velocity of the body is 7 m/s, given the displacement function \( s = 15t - 0.4t^2 \).

### Step 1: Understand the relationship between displacement and velocity
Velocity is defined as the rate of change of displacement with respect to time. Mathematically, this is expressed as:
\[ v(t) = \frac{ds}{dt} \]

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