Niki scored a 78 on her first test and an 85 on her second. Niki forgot what she got on her third test bit she knows that her average is an 85. What did she score on her third test? Write and sole an equation for this problem.

An average is written by the values / the amount of values.

We know there are 3 values, we know the numbers of 2, this can be modelled by the equation:

(78 + 85 + x) / 3 = 85

We solve that ^:

(163+x) / 3=85

163+x=255

x=92

The score on her 3rd test is 92