15% is 10% + 5% (or 0.15 = 0.1 + 0.05, dividing each percent by 100). Thinking about it this way is useful for two reasons. First, it's easy to multiply any number by 0.1; just move the decimal point left one digit.
Until the 4th visit, GWS has the better cost but after the 4th visit, the cost for Mega is higher.
