Since it is explicitly stated that 0.718 is the bigger number, then it is the minuend.
Subtrahend = x
Difference = 0.18
Equation:
0.718 - x = 0.18
x = 0.718 - 0.18
x = 0.538
The other number or subtrahend is 0.538.
To check:
0.718 - 0.538 = 0.18
0.18 = 0.18