convert percentages to units.
100% - 20% = 80% = 0.8
100% - 25% = 75% = 0.75
Then multiply those together:
0.8*0.75=0.6=60%
100% - 60% = 40%. That's the overall price percentage reduction.
P.S. You can convert percentages to units by diving % by 100 for example 100% divided by 100 is equal to 1.
And vice versa aka multiply units by 100 and you get %, for example 0.8 times 100 is 80%
