Question 118654
Look at the terms and see if you can determine the pattern. 
First term = 1.6
Second term = 1.6-0.4 = 1.2
Third term = 1.6-0.4-0.4 = 0.8
Fourth term = 1.6-0.4-0.4-0.4 = 0.4
I think you can see the pattern. 
Nth term = 1.6 - 0.4*(N-1)
Now that you have the formula, set it equal to the number to find N.
{{{1.6-0.4*(N-1)=-38.4}}}
{{{-0.4*(N-1)=-40.0}}} Subtract 1.6 from both sides of the equation. 
{{{N-1=100}}}Divide both sides by 0.4.
{{{N=101}}}
Check your answer with your original formula.
101st term = 1.6 - 0.4(101-1)=1.6-40.0=-38.4.
The answer checks out.