My brain has gone into meltdown and i need help with a formula that will look up data in two columns and if the criteria is met in column A it will sum the adjacent cells in column B

Column A has time elapsed from invoice date to payment received (in a number format, not date)
Column B has the value of said invoice

Using the criteria below i want a sum total of all invoices that were paid in each range. I can get it to do the sum for all invoices paid under 30days easily, but all the ones i've tried that include >=30<60 etc fail.
0-30
31-60
31-90
91-120
121+

Where am i going wrong? I've done it before but i just can't recall what i did...

any suggestions appreciated.