Till end-Jul 2020:
Credit card spend $500 -> 0.80%pa of $60k = $40
3 bill payment -> 0.35%pa of $60k = $17.5
From Aug 2020:
Credit card spend $500 -> 0.30%pa of $80k = $20
3 bill payment -> 0.30%pa of $80k = $20
If acct has $80k, and only do $500 credit card spend + 3 bill payment
(Existing) Till end-Jul 2020:
$(80k - 60k) -> 0.60%pa of $20k = $10
So for the presented case of $80k:
Total bonus interest (existing) = $40 + $17.5 + $10 = $67.50
Total bonus interest (from Aug 2020) = $20 + $20 = $40.00
Multiplier also 3k 