Everyone who sent in answers had to think about how to show the order of each little calculation. Some wrote down the answer after each step, some used the 'rule' that any multiplying and dividing should be done before the adding and subtracting, and some put brackets around bits that had to be done first.
All of the calculations shown below lead to 100 - but you could think about whether they have chosen the best way to write their answers.

Rayner Teo (Year 5, Tao Nan School, Singapore) sent in these two:
1 x 2 x 3 + 4 + 5 + 6 + 7 + 8 x 9 = 100
123 + 45 - 67 + 8 - 9 = 100

Christopher Block (Age 10, Tattingstone School)
"First of all I put down the numbers 1 to 9 on a bit of paper and kept adding, multiplying, dividing and subtracting until I got the right answer and these are what I came up with."
9 x 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 = 100
7 x 9 x 2 - 6 -5 - 4 - 3 - 8 x 1 = 100

Rowena Scott 1 + 2 + 3 + 4 +5 x 6 - 7 + 8 + 9 =100
I just tried some sums until I found one that worked.

Helena Manning (Age 10, Tattingstone School)
7 x 8 ÷ 4 x 6 + 1 + 3 + 5 + 9 - 2 = 100

Sally Hutton (Age 11, Tattingstone School)
1 x 2 + 3 x 4 x 5 - 6 + 7 + 8 - 9 = 100

Gina Allison and Kate Fletcher (Year 5, Yarm Primary School, Stockton-on-Tees)

9 x 5 = 45 + 8 = 53 x 2 = 106 −6 = 100 −7 = 93 +3 = 96 + 4 = 100 x 1 = 100

James Dixon (Year 5, Yarm Primary School, Stockton-on-Tees)

9 x 8 = 72 + 7 = 79 +6 = 85 +5 = 90 + 4 = 94 + 3 = 97 + 2 = 99 + 1 = 100.

Jessica Cattermole (Age 11, Tattingstone School, UK) "... I played around with the numbers ... until I had 500 which I could divide by 5."

9 x 8 x 7 = 504 + 3 = 507 + 4 = 511 +1 = 512 − 6 x 2 = 500 ÷ 5 = 100

Jason Day (Priory School, UK)
(1 x 2) + (3 x 4) x 5 +(6 + 7 + 8 + 9) = 100

Alexander Wilkinson (Age 9, Crofton Junior School, Kent)
[(1 + 2) ÷ 3] + 4 + (5 x 6) + (7 x 8) + 9 = 100

9 x 8 = 72 + 1 + 2 + 3 + 4 + 5 + 6 + 7 = 100

Daniel Loh (Age 10, Anglo-Chinese School, Singapore)
(1 + 2 + 3 + 4) x (((-5 + 6) x (-7 + 8)) + 9)
= 10 x (((-5 + 6) x (-7 + 8)) + 9)
= 10 x ((1 x (-7 + 8)) + 9)
= 10 x ((1 x 1) + 9)
= 10 x (1 x 9)
= 10 x 10
= 100

Lennox Akoto (Age 10, Norbury Manor Primary School)
(9x8) + (4x6) + 9 + 1 + 5 + 2 + 3 = 56 + 24 + 20 = 100

Declan Doyle & Harry Baker (Age 10, Norbury Manor Primary School)
(7 x 8) + (4 x 6) + 9 + 1 + 5 + 2 + 3 = 56 + 24 + 20 = 100