Q13.
There are 100 heads, so it means there are 100 rabbits and chickens altogether, as 1 animal has 1 head.
1 rabbit has 4 legs, and 1 chicken has 2 legs.
If all are chickens,
Total legs=100×2 =200
Diff (between total legs if all chickens and actual total) = 280-200=80
Difference in no of legs if 1 chicken is replaced by 1 rabbit =4-2=2
No. of rabbits=80÷2=40
No. of chickens=100-40=60

Q14.
1 bicycle has 2 wheels and 1 tricycle has 3 wheels.
If all are tricycles,
Total wheels=3×35=105
Difference (in no of wheels with all tricycles and actual no of wheels)=105-85=20
Difference in no of wheels if 1 tricycle is replaced by 1 bicycle=3-2=1
No of bicycles=20÷1=20
No of tricycles=35-20=15

This method is similar to guess and check, but it's called supposition :)