Jenny's total earnings:
total earnings = (hours worked on design)*40 + (hours worked on maintenance)*35
average earnings = total earnings / hours worked
So far she has
total earnings = 18*40 + 20*35 = $1420
which is an average per hour = $1420/38hr = $37.37
How many more hours on webdesign: x
so that
($1420+x*$40)/(38+x)=38
Now solve for x - this I let you do. My results is x=12 (you can verify by plugging it iback into the equation).
The answer is Jenny needs to work 12 more hours to claim an average earnings of $38/hr. Lmk if you have questions.