A boy playing with a pile of cubic blocks finds that he is short of $5$ blocks to make a solid whose base is $4\times 4$ square. Also he finds that if he makes a solid whose base is a $5\times 5$ square, he is left with an extra block. If the boy manages to make a solid where the base is a $3\times 7$ rectangle using all the available blocks, find the least number of blocks the boy mush have had.
I have solved this problem but I want to know what its answer is? My answer is 651
As already stated and explained in the comments, you answer is correct. You can get this result in Wolfram|Alpha using
ChineseRemainder[(-5,1,0),(16,25,21)].