At a national park in Kenya, an automatic camera photographs the number of leopards passing by

over a $1$ year period. Last year, the camera took $120$ photos of leopards, and it was determined that

the $120$ leopard photographs contained $55$ distinct $($different$)$ leopards.

How can we estimate N $,$ the total number of leopards living in the park? $($You can leave your answer

as an equation containing N $.)$