Answer:
First, two lines are perpendicular if one has slope m, and the other has slope -1/m (Not 1m as Alan says)
That is the first error of Alan.
Now, there is another error.
If m = 0, then -1/m tends to minus infinity (in this case we would have a vertical line).
But we can not really divide by zero, so we should state that:
m can not be zero, if a line is horizontal, the perpendicular line must be vertical.
If Ian uses 1/20 of the ketchup 4 times, this is equal to 4/20, or

.

can be simplified to

, which means we need to work out

of 12.2

is equivalent to 20%, which as a decimal is 0.2
0.2 x 12.2 = 2.44 - however, this is how much he has
used, and we need to figure out how much is left.
12.2 - 2.44 = 9.76
So, Ian has 9.76 oz remaining :)
You will have infinite solutions
Use the fact that
to solve it.

1st worker produced x pants,
2nd worker produced (x+8) pants.
x+x+8=86
2x=78
x= 39
x+8 = 39+8 = 47
1st worker produced 39 pants,
2nd worker produced 47 pants.