The rate of inflow of the salt per minute is 0.125 x 12 = 1.5 kg. But what about the outflow rate?

We will consider the outflow as separate from the inflow. Basically, the inflow rate of 1.5 kg per minute is fixed. We assume this event has passed.

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Now, we realise that the outflow rate is also 12 litres per minute. Suppose the amount of salt in the tank is q kg at the end of t minutes.

Do you agree with me that ideally the volume of solution in the tank always remains at 400 litres?

In other words, 400 litres of solution contain q kg of salt. If 12 litres were to flow out from this solution, by proportion, we realise that 12q/400 kg of salt will flow out.

(Sound off to me if this does not make sense to you).

This means that 12q/400, or 0.03q, kg of salt will flow out in every minute of 12-litre flow.

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Since the inflow rate is 1.5 kg (a fixed quantity)...
and the outflow rate if 0.03q kg (depending on what q is at that time)..

the overall flow rate is 1.5 - 0.03q.

If you don't understand my explanation for the show part, let me know and I will try to refine my explanation.

If you need me to do up the remaining parts, let me know as well and I will try and find the time to write my workings.

thank u, i got it :)