Answer: So we start with 888 -291.
Gillian does 888 - 300 = 588. so what she did there is round the 291 up to 300.
So the next steph would be make 588 - 9 = 581.
A better way to write this is
888 - 291 = 888 - 291 + 9- 9 = 888 - 300 -9 = 581.
In this equations always is useful to round up or down some number so its more easy the equation, en then subtract or sum the correspondent number.