You need 320 ml of a 65% alcohol solution. on hand, you have a 60% alcohol mixture. how much of the 60% alcohol mixture and pure alcohol will you need to obtain the desired solution?
you will need
ml of the 60% solution
and
ml of pure alcohol.

You will need

280 mL of the 60% solution

and  40 mL of pure alcohol.

Step-by-step explanation:

Let 'x' be the amount of 60% alcohol solution and y the amount of pure alcohol.

Therefore:

(0.6x + y)/320 = 0.65 ⇒ 0.6x + y = 208

x + y = 320

Solving the sistem of equations:

x = 280 and y = 40

Therefore, You will need

280 mL of the 60% solution

and  40 mL of pure alcohol.

there are lots of ways.   pick's theorem tells us if the vertices are lattice points the area a is

a = b/2 + i - 1

where b is the number of boundary lattice points and i is the number of interior lattice points.

in the figure i count b=16, i=50 so

a = 8 + 50 - 1 = 57

let's try the shoelace formula.   the area is half the sum of the cross products of the sides.  

a(-2,6)   -2(2)-6(-5) = 26

b(-5,2)   -5(-3) - 2(-2) = 19

c(-2,-3) -2(-3) - (-3)(4) = 18

d(4,-3)   4(3) - (-3)(5) = 27

e(5,3)   5(3) - 3(1) = 12

f(1,3)     1(6) - 3(-2) = 12

a(-2,6)

each of the cross products is twice the area of the triangle with the associated side and one vertex the origin. so the area of abo=26/2=13.

area = (1/2) (26 + 19 +   18 + 27 + 12 + 12) = 57

that checks!