If an initial quantity continuous decay at an exponential rate , then the final amount after a time is given by the following formula .

(**a**)

Find proportion of words remains unchanged.

The function is .

is the proportion of words that remain unchanged.

is the time since two languages diverged.

is the rate of replacement.

Since two languages diverged years ago .

Since rate of replacement is , .

Substitute and in .

.

Proportion of words remains unchanged is .

(**b**)

Find how many years will only 1% of the words remain unchanged.

Since 1% of the words remain unchanged, .

Substitute and in .

Apply logarithm on each side.

.

1% of the words remain unchanged in years.

(**a**) Proportion of words remains unchanged is .

(**b**) 1% of the words remain unchanged in years.

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