substitute in:
15y = 4 + 5 * ( (2/3) y - (1/3) )
15y = 4 + (10/3) y - (5/3)
15y - (10/3) y = (7/3)
(35/3) y = (7/3)
35 y = 7
y = 1/5
when y = 1/5
x = (2/3) (1/5) - (1/3)
x = (2/15) - (1/3)
x = -1/5
=============
5)
(6/100) x - (1/100)y = 6/10
(22/100)x + (13/100)y = 122/10
6x - y = 60 =====> * 13
22x + 13y = 1220
78x - 13y = 780
+
22x + 13y = 1220
-----------------------
100x + 0 = 2000 ====> x = 20
when x = 20
6*20 - y = 60
120 - 60 = y
60 = y
============
6)
$1 ------- call it -----> x
$10------ call it -----> y
has a total of 59 bills -----> x + y = 59
combined monetary value of $239
1x + 10y = 239
x + y = 59
- <-- minus
x + 10y = 239
--------------------
0 - 9y = -180 =====> y = 20
when y = 20
x + 20 = 59
x = 59 - 20
x = 39
==========
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