# Exponential Growth and Decay Help? (1 Viewer)

#### Heresy

##### Member
I need help with part c (mainly with how fast the salt is dissolving) and d please, any help would be appreciated. Thanks.

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#### integral95

##### Well-Known Member
When you're finding the rate, or how fast or slow something is, you use the derivative expression.

so for c)
you find dS/dt and then sub in t = 5 and your value of k from part b), that's your rate.

d)sub S = 4 (as that's the amount that's left undissolved ) and your k from b)

Then you solve for t, assuming you have you the logarithm knowledge to work out the algebra.

#### Heresy

##### Member
When you're finding the rate, or how fast or slow something is, you use the derivative expression.

so for c)
you find dS/dt and then sub in t = 5 and your value of k from part b), that's your rate.

d)sub S = 4 (as that's the amount that's left undissolved ) and your k from b)

Then you solve for t, assuming you have you the logarithm knowledge to work out the algebra.
Thanks.