They have 24 credits. They need 64 credits. They need 64 - 24 = 40 more credits
If the question is "what percent of the total credits do they still need?" then:
Since they need 40 more credits, the percent of the total credits they still need is (40/64) * 100 = 62.5%
If the question is "what percent more than what they already have do they need?" then:
since they need 40 more credits and they already have 25 credits, they need (40/25) * 100 = 160% more credits than what they have now.