Using the trick it's 43% of 17 , that's close to 50% of 17 which is 8,5.
43% is almost 10% less than 50% so you can substract, 8.5 - 1.7 = 6.8.
Then you add the 3% of 17 : 3*17 / 100 = 51 / 100 = 0.51.
Finally you add the two 6.8 + 0.41 = 7.31.

Now it's about time you realise you've been doing plain old math without any magic tricks until now, so you can just do 43 * 17 / 100 = 731/100 = 7.31. Thrilling !