b. A boat travels 24 miles upstream in 3 hours. It then takes 3 hours to go 36 miles downstream. Find the speed of the boat in still water and the speed of the current.
let speed of boat in still water be x,and speed of current downstream be y
then upstream the speed of the boat will be (y-x) miles per hour
distance=24 miles in 3 hours at speed of (y-x)
speed=(y-x)=distance/time=24/3=8
speed downstream is (x+y)
speed=(x+y)=dstnce/time=36/3=12
so we have 2 equations and 2 variables,
y-x=8 AND x+y=12
y=8+x AND y=12-x
y=8+x=12-x
8+x=12-x
x+x=12-8
2x=4
x=2
so speed of boat is 2 mph
y=8+x=10 mph which is speed of current

c. A service station wishes to mix 40L of a 78% antifreeze solution. How many liters of a 75% solution and a 90% solution should be used in forming the mixture?
let x litres be taken of the 75% solution
let y litres be taken of the 90% solution
then (x+y)=40 as we need 40litres of the solution
so y=40-x
now we can write our equation as,
x(75%)+y(90%)=40(78%)
x(75/100)+y(90/100)=40(78/100)
75x+90y=40(78)
75x+90y=3120
75x+90(40-x)=3120
75x+90(40)-90x=3120
-15x+3600=3120
-15x=3120-3600
-15x=-480
x=32
y=40-x=8
so you need 32 litres of 75% solution,and 8liters of 90% solution

Hope this helps,
-xC