Question 882716
You start with 70. 
When will it be 35?
You need a negative exponent or else the amount of material increases.
{{{35=70e^(-0.19t)}}}
{{{1/2=e^(-0.19t)}}}
{{{-0.19t=ln(1/2)}}}
{{{t=ln(1/2)/(-0.19)}}}
{{{t=3.64}}} hours