'is' in mathematics means equal
'of' in mathematics means multiplication
Translating 864 is 40% of what number into an equation, we get:
864 = 40% * x
40%, or 40 percent, is nothing but 40 over 100 or 40/100, which is the same as 2/5.
864 = 2/5 * x
Multiply 5 on both sides.
864 * 5 = 2/5 * 5 * x
4320 = 2x
Divide 2 on both sides
4320/2 = 2x/2
2160 = x
x = 2160
864 is 40% of 2160.