Any one of them or two of them or all of them can solve the problem
For X to solve – 1/4 so, not solve – 3/4
For Y to solve – 2/3 so, not solve – 1/3
For Z to solve – 3/5 so, not solve – 2/5
So, the required probability is = solved by only x + solved by only y + solved by only z + solved by x and z + solved by y and z + solved by x and z + solved by x, y and z
= 1/4 * 1/3*2/5 + 2/3*3/4*2/5 + 3/4*1/3*3*5 + 3/4*1/3 * 3*5 + 2/3*3/5*3/4 + 1/4*3/5*1/3+1/4*2/3*3/5
= 2/60 + 12/60 + 9/60 + 9/60 + 12/60 + 3/60+6/60
= 51/60 = 17/20 = 0.85