number of pens for Bob --- x
number of pens for Jane --- y
case1: Bob gives 3 pens to Jane
Bob has x-3
Jane has y+3
so x-3 = y+3
x-y = 6
case2: Jane gives 3 pens
Bob has x+3
Jane has y-3
x+3 = 3(y-3)
x+3 = 3y - 9
x - 3y = -12
subtract the two equations
2y = 18
y = 9
sub into 1st equation:
x-9 = 6
x = 15
Bob now has 15 and Jane has 9
after 1st transfer, bob has 12 and Jane has 12, good!
afer 2nd transfer, Bob has 18 and Jane has 6
does he have 3 times what she has? YES!
all is good!
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