Jerry solved this equation: 3 ( x − 1
4
) =
13
6

1. 3x −
3
4
=
13
6

2. 3x −
3
4
+
3
4
=
13
6
+
3
4

3. 3x =
26
12
+
9
12

4. 3x =
35
12

5. (
3
1
)
3
1
x =
35
12
(
3
1
)

6. x =
105
12

answer:  

\boxed {\frac{1}{5}}  

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solution-

60 *   \frac{1}{5}   = 12

90 * \frac{1}{5} = 18

75 * \frac{1}{5} = 15

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

hence, the answer would be \boxed {\frac{1}{5}}