Combine like terms and solve for "f"...
f+0.4+0.6f=-0.6........given, problem
statement
1.6f+0.4=-0.6........combine like terms
1.6f=-1............subtract 0.4, both sides
∴ f=-0.625....divide both sides by 1.6
Let's check our solution...
-0.625+0.4+ 0.6(-0.625)=-0.6
-0.625+0.4+(-0.375)=-0.6
-0.6=-0.6...true, √check
Our solution is correct. Always check your solution and work!
