How much is 85% of 51 months ? Or how do I get the answer in math terms

To obtain the answer you would essentially have to mulyiply 51 x 0.85 (the equivalent of 85% in decimal form). The answer would be 43.35 months. This is about 1300.5 days (43.35 x 30, note that this is merely an approximation).

85 percent is 0.85

0.85 x 51
= 43.35 Months
or
43 months and 10 days approximately.

multiply 51 by .85