It's a trick question and an impossible sum. First you have to convert the number into millimetres, and the percentage into a fraction. So we now have one tenth of 900 millimetres. If you have a 12 inch ruler, you wil see that 900 millimetres isn't on it. Erego, ten one hundredthsof 900 is still 90, leaving you with the same amount you started with.
55
100.
18
10000.
That would be 16.
78-90cm converted into inches = -12
10% of 350 is 35.
10% of 190 is 19.
10% of 8,100 is 810.
10% of 6.5 is 0.65
10% of 250 is 25.
52 pence !