$$F=M-4$$  $$m-4+M=10$$  $$M=7$$ 
Felix and Marmalade are two cats. Together they weigh $$10\text{kg}$$.  Felix weighs $$4\text{kg}$$ less than Marmalade. How much does Marmalade weigh? 
Options:
(a) $$3\text{kg}$$ 
(b) $$6\text{kg}$$ 
(c) $$7 \text{kg}$$ 
(d) $$9\text{kg}$$ 
(e) $$14 \text{kg}$$ 