Explanation:
Assume the number of double matches is x (matches)
the number of single matches is y (matches)
(Condition: x > 0 and y > 0)
Because there are 13 matches in progress, so:
x + y = 13 (1)
Because there are 38 people playing (4 people on each double match and 2 people on each single match), so:
4x + 2y = 38 (2)
A system of equation is made by combining (1) and (2):
x + y = 13 (1)
4x + 2y = 38 (2)
Take (2) - 2×(1):
=> 2x = 12
=> x = 6
Substitute the result on (1)
=> y = 7
So there are 6 double matches and 7 single matches being played