Nested if formula for bonus determination

I have a sheet that tracks the income of commission employees. It is calculated monthly. the employees have a commission of "x" until they reach 25k of commissions. When the commission rises to 35K it increases to "y". at 40K it increase to "z" . at 50K it increases to "a" finally at 60K, it increases to "b". the higher level commission is earned only after the employees surpass the various thresholds. IE, they don't earn the higher commission if a comission earned at a lower threshold crosses an upper threshold.. This accumulates during the year and then resets anew at the beginning of the next year. I need a formula that will look at the income in cell B^ and calculate the commission earned. Then in C6 onward, it will add B6 to C6 to see if the higher commission has been achieved. This keeps going to the end of the year.