Question about minimum allowed raise

[maddo]

Hi.

Blinds are 25/50.

2 players (Fred and Bob) see the flop.

Fred checks.
Bob bets 150.
Fred wants to raise the minimum. Can he raise to 250 (another 100) because Bob's bet was 100 more than the minimum (50). Or does he have to raise to 300 (double Bob's bet)?


Cheers.