If 1960 = 2^a.5^b.7^c, calculate the value of 2^-a.7^b.5^-c.

If a = x^{m + n}.y^l; b = x^{n + l}.y^m and c = x^{l + m}.yⁿ,

prove that : a^{m - n}.b^{n - l}.c^{l - m} = 1