Formula :
F(fine) = $0.25+0.05 d (days)
A book is fine is $1.30 . How may days book was due.
My work
1.30=0.25+0.05d
-0.25=0.05d
1.05= 0.05d
1.05/0.05=d
21=d
I get 21 days is this right?
I agree with your answer but not your math steps. For example, step 2 is not right because -0.25 is not equal to 0.05d.
1.30=0.25+0.05d
1.30 - 0.25 = 0.05d
1.05 = 0.05d
1.05/0.05 = d
21 = d