>>21580768

(c+r) + (d+r) + (c+r) = 10 + 20 + 24

2c + 2r + 2d = 54

(2c + 2r + 2d) ÷ 2 = 54 ÷ 2

c + r + d = 27

>>21581038

oops, typo

(c+r) + (d+r) + (c+d) = 10 + 20 + 24

2c + 2r + 2d = 54

(2c + 2r + 2d) ÷ 2 = 54 ÷ 2

c + r + d = 27