Here is the question:
If the tax rate is 25% and net income is $100,000, what is gross income?
If the tax rate is 25% then the tax on gross income, G, is 0.25G. The net income, N, is the gross income minus that tax: N= G- 0.25G= (1- 0.25)G= 0.75G.
So to go from gross income to net income, you multiply by 0.75.
To go the other way, from net income to gross income, do the opposite- divide by 0.75.