Let Number of red counters be X.
Let Number of blue counters be Y.
ratio of red counters and blue counters: X/7 = Y/3 ----(1)
If 36red counters are removed and 24blue counters added, there will
be an equal numbers of red counters and blue counters:
X - 36 = Y + 24 -----(2)
(1): 3X - 7Y = 0
(2): X - Y = 60 --------> 3X - 3Y = 180
Sub (2) into (1):
(180 + 3Y) - 7Y = 0
4Y = 180
Y = 45
Sub Y=45 into (2):
==> X - 45 = 60
==> X = 105
