Answer:
Total money earned on Thursday is $1090.
Step-by-step explanation:
Let amount of money for one house call of Plumber A be x
So, amount of money for 2 house call of Plumber A=2 x
So, amount of money for 4 house call of Plumber A=4 x
So, amount of money for 3 house call of Plumber A=3 x
Let amount of money for one house call of Plumber B be y
So, amount of money for 8 house call of Plumber B=8 y
So, amount of money for 7 house call of Plumber B=7 y
So, amount of money for 9 house call of Plumber B=9 y
Let amount of money for one house call of Plumber C be z
So, amount of money for 8 house call of Plumber C=8 z
So, amount of money for 10 house call of Plumber C=10 z
So, amount of money for 9 house call of Plumber C=9 z
Now we are given that The total amount of money earned by all three plumbers on Monday was $1,400. 
On Monday Plumber A has 2 calls , Plumber B has 8 calls and Plumber C has 8 calls
So, equation becomes: --1
  --1
On Tuesday they earned a total of $1,660,On Tuesday Plumber A has 4 calls , Plumber B has 7 calls and Plumber C has 10 calls.
So, equation becomes: ---2
 ---2
On Wednesday, they earned a total of $1,650.On Wednesday Plumber A has 3 calls , Plumber B has 9 calls and Plumber C has 9 calls.
So, equation becomes: ----3
 ----3
Solving equation 1 ,2 AND 3 
Equation 3: 
                    ---4
   ---4
Now substitute the value of x from 4 in 1 and 2
 and
 and 
 and
 and 
 --- (a) and
 --- (a) and  ---(b)
---(b)
Now substitute the value of y from a in b






Substitute the value of z in (a)
 
 
 
 
 
 
 
 
Substitute the value of y and z in 1





So, amount of money for one house call of Plumber A = $100
So, amount of money for one house call of Plumber B =$80
So, amount of money for one house call of Plumber C =$70
On Thursday Plumber A made four house calls, Plumber B made six house calls, and Plumber C made three house calls.
So, equation becomes : 
Substitute the values of x ,y and z
 
 
 
Thus total money earned on Thursday is $1090.
Hence Option C is correct.