Question 145088
Let's denote Milk carton by M,
Doughnuts by D
Coffee by C


First Equation formation:
they spend $1.85 on 1 carton of milk, 2 donuts, and 1 cup of coffee
So with the above said first equation becomes: 1M+2D+1C=1.85

Second Equation formation:
next day, they spend $2.30 on 3 donuts and 2 cups of coffee.
So with the above said second equation becomes: 3D+2C=2.30

Third Equation formation:
third day, they bought 1 carton of milk, 1 donut, and 2 cups of coffee and spent $1.75
So with the above said second equation becomes: 

Now we solve these 3 equations:
1M+2D+1C=1.85-------------------------->eqn 1
3D+2C=2.30       ---------------------------->eqn 2
1M+1D+2C=1.75--------------------------->eqn 3

Look for similar number of M's, D's or C's in any 2 equations. we see that Equation 1 and 3 have same numbers of M's that is "1M"
Now subtract Equation 1 from 3 or 3 from 1 whichever you find easy.
Let's say we subtract 3 from 1, so we get:

1M+2D+1C=1.85
1M+1D+2C=1.75
-----------------------
(1M-1M)+(2D-1D)+(1C-2C)=(1.85-1.75)
implies 0M+1D-1C=0.1---------------------->eqn 4

Now we will try to solve eqn 2 and eqn 4.
3D+2C=2.30       ---------------------------->eqn 2
1D-1C=0.1---------------------->eqn 4
From eqn 4 we have D=0.1+C------>eqn 5
Now putting this value of D in eqn 2, we have
3(0.1+C)+2C=2.30
implies 0.3+3C+2C=2.30
implies 5C=2.30-0.3
implies 5C=2
implies C=0.4

Now from eqn 5, D=0.1+0.4=0.5
So, D=0.5

From eqn 1, M=1.85-2D-1C
implies M=1.85-2*0.5-0.4
implies M=1.85-1-.0.4
So, M=0.45

Fourth day cost: 2M+2D=2*0.45+2*0.5
implies fourth day cost=0.9+1=1.9

So they won't be able to get 2 Dougnuts and 2 cartons of Milk with $1.8