1) If A= C and B = A then B = C.
When you (A) are truly in love (C being "romantic feelings") with a certain someone (B) and they KNOW how much you care about them but only mutter back "ditto" (or something of the sort), don't overlook it. Men often shy away from saying that word but it doesn't mean that they don't feel the same.
2) A common mathematical miscalculation that men make (often around Valentine's Day) is the assumption that 'y' is directly proportional to 'x' (when y increases, x increases). In this case, y = gifts and x = the number of sexual favors they expect you to give them in return. But any woman knows that half a dozen roses, a fancy dinner, a box of chocolates, a piece of jewelry, and a thoughtful card does NOT translate into "doing it" 10 times. HELL NO.
3) On the other hand, y IS inversely proportional to x in another context. (As y increases, x decreases.) Women are somehow preprogrammed with the knowledge that the sexier they dress (y = sexiness rating) the less time it takes for a man to want to sleep with them right then and there. This can work to our advantage because we then hold all the power.