Answer: 35 children
Explanation:
Let the number of children who play only football be f , the number of children who play only
tennis be t and the number of children who play both sports be b.
Since there are 42 children, f + t + b = 42.
Also, since the number of children who play tennis is equal to the number of children who play
only football, t + b = f . Therefore f + f = 42. So f = 21 and t + b = 21.
Finally, twice as many play both tennis and football as play just tennis. Therefore b = 2t.
Substituting for b, gives t + 2t = 21. Hence t = 7.
Therefore the number of children who play football is 42 − t = 42 − 7 = 35.