Question

by what number should we multiply 3/5 so as to get -1?

Answer 1

let the number be 'X'
3/5 × X = - 1
3X = - 5
X = - 5/3
so , the number is - 5/3
hope it helps;)

Answer 2

let consider the incognito number as a variable like 'x'
means it is said that by multiplying 3/5 and x we get -1
so 3/5(x)=-1
    =x=-1/3/5
        =-1(5/3)
        =-5/3