Arecipe for 5 people uses 250g of flour and 75ml of milk.

a) how much flour and milk will be needed for 8 people?
b) how much flour and milk will be needed for 3 people?

One way to solve this is to find how much flour and milk is needed for one person. You divide 5 people by 5 to get one person, so you also divide 250g and 75 ml by 5. For one person, 50g of flour and 15ml of milk is needed. You will use this to find a) and b). 

a) For eight people, you multiply everything by 8.
50g * 8 = 400g
15ml * 8 = 120ml

b) For 3 people, multiply by 3.
50g * 3 = 150g
15ml * 3 = 45ml

Another way to do this is to use proportions.
a)
5 people          8 people
=
250 g Flour           x
Now cross multiply:
5x = 200
x = 400g of Flour

5 people       8 people
=
75 ml Milk         x

5x = 8*75
5x = 600
x = 120 ml of Milk