First need to know what limits you are playing. to know what player A's raise was ex: if your playing 5/10 then player a raised $30 on top which would mean player b's raise was more than half of player a's bet. Posty the limits you are talking about so we can give you the proper answer. But if your talking about post flop then player c must make a full raise which is $40 so the raise should be to $80. Heres the rules and a few examples from Roberts rules of poker:
2. The minimum bet size is the amount of the minimum bring-in, unless the player is going all-in. The minimum bring-in is the size of the big blind unless the structure of the game is preset by the house to some other amount (such as double the big blind). The minimum bet remains the same amount on all betting rounds. If the big blind does not have sufficient chips to post the required amount, a player who enters the pot on the initial betting round is still required to enter for at least the minimum bet (unless going all-in for a lesser sum) and a preflop raiser must at least double the size of the big blind. At all other times, when someone goes all-in for less than the minimum bet, a player has the option of just calling the all-in amount. If a player goes all-in for an amount that is less than the minimum bet, a player who wishes to raise must raise at least the amount of the minimum bet. For example, if the minimum bet is $100, and a player goes all-in on the flop for $20, a player may fold, call $20, or raise to at least a total of $120
3. All raises must be equal to or greater than the size of the previous bet or raise on that betting round, except for an all-in wager.[ Example: Player A bets 100 and player B raises to 200. Player C wishing to raise must raise at least 100 more, making the total bet at least 3. A player who has already checked or called may not subsequently raise an all-in bet that is less than the full size of the last bet or raise. (The half-the-size rule for reopening the betting is for limit poker only.)
4. Multiple all-in wagers, each of an amount too small to qualify as a raise, still act as a raise and reopen the betting if the resulting wager size to a player qualifies as a raise.
Example: Player A bets $100 and Player B raises $100 more, making the total bet $200. If Player C goes all in for less than $300 total (not a full $100 raise), and Player A calls, then Player B has no option to raise again, because he wasn't fully raised. (Player A could have raised, because Player B raised.)