Plz me i don’t get these i’m hoping for a smart math

probability of building + probability of garden + probability of monument = 1

                   2/5            +             3/10                 +            x                             = 1

                  2(2)            +             1(3)                  +         10(x)                           =10(1)

                    4              +              3                    +           10x                           =10

                                                                                        10x                           = 3

                                                                                           x                           =3/10

Total cards (c) x  probability of monument (3/10) = # of monument cards (9)  

3/10c = 9

c = 9(10/3)

c = 30 Total postcards in the box

Total cards (30) x  probability of building (2/5) = # of building cards (b)

30 x 2/5 = b

12 = b  number of building postcards

Total cards (30) x  probability of garden (3/10) = # of garden cards (g)

30 x 3/10 = g

9 = g  number of garden postcards

Answers: 30 postcards, 12 building cards, 9 garden cards

4)

Total cards (c) x  probability of building (2/5) = # of building cards (12 + 8)

c (2/5) = 20

c = 20(5/2)

c = 50

Total cards (50) x  probability of garden (3/10) = # of garden cards (g)

50 (3/10) = g

15 = g

There were 9 garden cards and now there are 15

Total cards (50) x  probability of monument (3/10) = # of monument (m)

50 (3/10) = m

15 = m

There were 9 monument cards and now there are 15 cards

 6 more garden and 6 more monument postcards

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