£400-15% is your equation you use, but I will use the equation of part/whole =percent/100, where a is the variable for the part, w is the variable for the whole (£400) and p is the percent that we are trying to find or use (15%):
a/400 = 15/100; multiply: 400•15= 6000; then divide: 6000/100=60
So we now know that the answer to your problem is that the Charity receives £60
