# Doubt in understanding logic

watched the video many times but not able to understand logic

@simi_1188,
This will happen way more frequently as you progress through harder problems. Just leave it for now, and try some simpler problems on dp, Come back to it again latter.

i have already solved min steps and fibonacci which more simpler problems i can do in which if doubt arises i can ask the TAs for help

@simi_1188,
Dp is way more overwhelming topic to be intimidated by a count of â€ś2â€ť problems solved for experience.
And you can ask us doubts any time you want, but if a dedicated detailed video made by bhaiya couldnâ€™t help you, we hardly could. But obviously we will be happy to try.
So, what is it you donâ€™t understand in Bhaiyaâ€™s explanation?

can u give a brief idea how calls are being made

@simi_1188,
Focus on the state tree bhaiya made at the very beginning,
There are three possible ways to reach a sum of 15 Rs.

1. You already have 14 Rs. and you pick up a coin of Rs. 1.
2. You already have 8 Rs. and you pick up a coin of Rs. 7.
3. You already have 5 Rs. and you pick up a coin of Rs. 10.

Out of all these possibilities you want to choose the one which yeilds you minimum coins.
Thus recurrence, dp[15] = 1+min{dp[14],dp[8],dp[5]}
And in generality, dp[i]=1+min{dp[i-coins[j]]}, where dp[i]=minimum coins to have i Rs.

So calls simply would be standing on state i, call all the states i-coins[j], and take their minimum.

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