The number of fish,  N, in a pond is decreasing according to the model N(t)=ab−t+40,   t(greater than or equal to)0 where a and b are positive constants, and t is the time in months since the number of fish in the pond was first counted. At the beginning 840 fish were counted. After  4 months 90 fish were counted. Find the value of b.

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Differential Equations

Have you ever observed the movement of the satellites and rockets or how the planets go around the sun in their orbits? How will you determine their motion? Definitely they follow some scientific laws and these kinds of problems are framed as differential equations.