If you did 5g daily, this would end up doing a residual change of 40% weekly. The same result will be true if you do a 20g change once a week and 3 5g changes. Just to show I'm not just dividing it by 75, if you take 40% of 75, you get 30, with your first plan, you end up changing 45 gallons weekly, and the second plan you change 35 gallons weekly. Now... how on earth does 30 gallons, 35 gallons, and 45 gallons all equal a 40% weekly change? Its all about residuals. When you replace the water you have removed, you are diluting the waste which remains in the tank. Once you take more water out of this, you're removing (assuming a perfect world, this is a simple gauge, not exact) mostly polluted water, but some percent of the water you are removing will be from whatever was most recently put into the tank. It doesn't QUITE work out that way, but that's a real basic overview.
The easiest way to calculate your residual change per week is to take the decimal equivalent of whatever percent of water is still remaining in the tank after siphoning, but before refilling. Example: Lets say I want to change 10 gallons three times in one week. 10/75 ~ 13%. So, I'd take .87 (remember, decimal equivalent of whatever is REMAINING in the tank, not whatever is taken out) and multiply that by .87, and multiply their product by .87. This will leave you with .65 (rounded) displayed on the calculator, meaning you've changed 35% of the residual water that week. I myself would recommend aiming to hit 50% residual weekly, but I myself am partial to overkill water changes. (I keep discus)
Once you get a water change routine nailed down, it'll become second nature and it'll become as easy (though often as trivial) as putting your socks on in the morning. Your fish will certainly love you for it.