Okay so the answer is 50(1.05)t because essentially you are recieving 105% back, 100% of what you deposited plus the interest which is 5%. So 1=100% thus 105 would be 10500% which is WAY off. And then 5=500% interest and that's off. And then with just 50(0.5) You are not including in the fact that you still have that 50$, it would be like getting only 25$ instead of keeping the 50$ plus whatever is gained in interest.