How many numbers below 100 can be expressed as the difference of two perfect squares in only one way?

1) 24
2) 25
3) 34
4) 35
5) 36

• perfect squares within 100 are 4,9,16,25,36,49,64,81.
  there are 8 numbers.
  each element can be subtracted only once.so,we use 4(8 times) i.e( 81-4,64-4,49-4,.......) and 9 (7 times) and 16 ( 6 times)..and so on
  the no. of elements are therefore(counting times) =8+7+6+5+4+3+2 (1 cannot be used coz, we need 2 numbers to find difference)
  =35 numbers
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