Question 1128300
I do not know of a formula,
and different teachers may use different formulas.
I believe formulas are for computers,
who cannot think for themselves.
Humans should be able to reason their way to their own formulas.
 
We know that
water entered the lake
X) from rain, and 
A) from the creeks flowing into the lake, and
water exited the lake
B) through evaporation, and
C) through the Butas River.
That caused
D) an increase in the amount of water in the lake.
We can figure that
{{{X+blue(A)-green(B)-blue(C)=red(D)}}}
(water entering - water leaving = water increase in the lake).
 
It would make sense to express all those amounts of water in cubic meters.
That would take some calculations, because
the water entering and exiting through creeks and river is measured in cubic meters per second,
while the precipitation and evaporation are given in mm, apparently as total for the whole month.
 
The surface of the lake in square meters is
{{{(82km^2)(1000m/km)(1000m/km)="82,000,000"}}}{{{m^2}}}
The level of the lake rose by
{{{100mm(1m/"1000 mm")=0.1m}}}
The water increase in the lake, in cubic meters can be estimated as
{{{red(D)=(82km^2)(100mm)="82,000,000"}}}{{{m^2(0.1m)=red("8,200,000")}}}{{{m^3}}}
The evaporation from the lake, in cubic meters can be estimated as
{{{green(B)=(82km^2)(900mm)="82,000,000"}}}{{{m^2(0.09m)=green("7,380,000")}}}{{{m^3}}}
 
The amounts of water coming in from the creeks and leaving through the Butas River are given in cubic meters per second,
and we have to calculate for a 31-day month with 24-hour days, and
{{{31day(24hour/"1 day")(60minute/"1 hour")(60seconds/"1 minute")="2,678,400"}}}{{{seconds}}}
The total inflow in cubic meters can be calculated as
{{{blue(A)=(35.9m3/"1 second")*"2,678,400"}}}{{{seconds=blue("96,154,560")}}}{{{m^3}}}
The total outflow in cubic meters can be calculated as
{{{blue(C)=(40.2m^3/"1 second")*"2,678,400"}}}{{{seconds=blue("107,671,680")}}}{{{m^3}}}
 
Expressed all in cubic meters, {{{X+blue(A)-green(B)-blue(C)=red(D)}}} looks like
{{{X+blue("96,154,560")-blue("107,671,680")-green("7,380,000")=red("8,200,000")}}}
{{{X=red("8,200,000")-blue("96,154,560")+blue("107,671,680")+green("7,380,000")="27,097,120"}}}
That volume of water, expressed as precipitation falling over the
{{{82km^2="82,000,000"}}}{{{m^2}}} of lake surface is
{{{27097120m^3/82000000m^2=0.33045m=about}}}{{{highlight(330mm)}}}