We will use the condition "85% of these biplanes have a speed not greater than 131 miles/hour"
to find the raw score on this specified normal curve, that corresponds to 85% of these biplanes.


  +---------------------------------------------------------------------------------------+
  |  This score, when expressed/measured as a raw score on the specified normal curve,    |
  |  has its specific value; but when measured in terms of the standard deviation,        |
  |  it is the same as the standard z-score of the 85% area on the standard normal curve. |
  +---------------------------------------------------------------------------------------+


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      / It is the major knowledge you need to know when solving such problems. /

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So, we first determine the standard z-score on the standard normal curve, using the condition
that 85% of the area under the standard normal curve is on the left of this standard score.
So, use the invNorm function of the graphing calculator in this format

                               area    mean  SD

    standard z-score = invNorm(0.85,    0,    1) = 1.036433.


Now, we know that this standard z-score of 1.036433 is the same as the raw z-score
expressed in terms of the standard deviation

    1.036433 =  = .


In this equation, only SD (the standard deviation) is the unknown, so we easily find it

    1.036433*SD = 131-117.9 = 13.1

             SD = 13.1/1.036433 = 12.6395   (rounded).


ANSWER.  The standard deviation is 12.6395 miles.