My initial reaction is to try to isolate Q by taking it over to 30 so the equation would be 90-0.8=30/Q
The idea behind what you did is good, but the execution is flawed because you performed an "illegal" operation. You can't divide just one term in an expression - it's an all or nothing package deal. Consider the following:
2x=8
If we wanted to know the value of
x, we could divide both sides by 2, yielding:
22x=28⟹x=4
Now, consider a slight variant on that exercise:
2x+1=9
Let's proceed with the exact same type of operation you did in your post, and divide only the
x term by 2:
22x+1=29⟹x+1=29⟹x=27
We can then plug this back in to check our answer:
2(27)+1=7+1=8=9
Oh dear, that's not right at all! But where did we go wrong? Recall what I said earlier about division. What we should have done is:
22x+1=29⟹x+21=29⟹x=28=4
Ah, much better, that worked exactly as we intended.