PSAT math questions (3rd) help

3rd part of stumbling upon questions I don't get. Eeee ><;

-if 5a=3 and 3b=5, what is the value of a/b?

-Micheal gave Sally 1/3 of his baseball cards and Heidi took 1/4 of WHAT WAS LEFT. What fraction of his cards did Micheal have left?

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Answers and clear explanations are appreciated!

5a=3 and 3b=5

first one divide both sides by 5 to get
a = 3/5

second one divide both sides by 3 to get
b = 5/3

a/ b = (3/5) /(5/3)

multiply it by its reciprocal
3/5 * 3/5
a/b = 9/25


2.) michael has x cards
sally took 1/3
so he has (2/3)x cards left

heidi took 1/4 out of those (2/3)x cards
1/4*2/3 = 1/6

2/3 - 1/6 = 3/6 = 1/2

he has 1/2 of his original cards

1.) If 5a = 3,
a = 3/5

If 3b = 5,
b = 5/3

a / b = (3/5) / (5/3)
a / b = (3/5)(3/5)
a / b = 9/25
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2.) Remainder = x

x = 1/4 of 1/3
x = (1/3)(1/4)
x = 1/12

Michael had 1/12 of his cards left.
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1. 9/25
2. 1/2