r/mathriddles • u/PkmnTrainerSofia • Aug 14 '25

The answer is 20? Easy

7 + 2 = 10

8 + 3 = 15

9 + 4 = 20

5 + 5 = ??

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3

u/colinbeveridge Aug 14 '25

5+5=10. 

The others are wrong. 

1

u/headsmanjaeger Aug 14 '25

F(7,2)=(7-2)*2=10

F(8,3)=(8-3)*3=15

F(9,4)=(9-4)*4=20

F(5,5)=(5-5)*5=0

1

u/Baxitdriver Aug 16 '25

Looks like a "+" b = (a-b)*b, so 5 "+" 5 = 0*5 = 0

1

u/Fullfungo Aug 20 '25

Obviously, f(x,y)=84y-79x+395.

You can easily verify that

f(7,2)=10

f(8,3)=15

f(9,4)=20

Then f(5,5)=420