The starting number that Grace thought of was 20.
Let the number thought by Grace (that is the starting number) be x.
11 is added to the number, to get:
x + 11
It is then multiplied by 5, to get:
5(x + 11)
15 is then taken away to get:
5(x + 11) - 15
The result is divided by 7 to get 20, hence:
[5(x + 11) - 15]/7 = 20
5(x + 11) - 15 = 140
5x + 55 = 155
5x = 100
x = 20
Therefore the starting number that Grace thought of was 20.
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