Solve by model :heutistmeintag wrote:oh i see, now I can solve by alegbra.

B=20+G

1/3G+1/4B=19

4G+3B=19*12=228

4G+3*(20+G)=228

4G+60+3G=228

7G=228-60

7G=168

G=24

B=20+24=44

Total number of children invited is (24+44)=68

ok, can someone explain using modelling?

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Boys : 12u + 20

Girls : 12u

1/4 boys did not come -> 3u + 5 (you can break 20 into 4 parts, each part is 5)

1/3 girls did not come -> 4u

7u + 5 = 19

1u = 2

Altogether -> 24u + 20 = 48+20 = 68.