4. In the equality $a+b=c+d=e+f$ the letters represent different prime numbers less than 20. Determine at least one solution.
4. All prime numbers less than 20 are: 2, 3, 5, 7, 11, 13, 17, and 19.

$1 \text{POINT}$

There are 28 possible sums in total:

$2+3=5$

$2+5=7$

$3+5=8$

$2+7=9$

$2+11=13$

$2+13=15$

$2+17=19$

$2+19=21$

2 POINTS

Among them, only the sum 24 appears 3 times.

$1 \text{POINT}$

One possible solution (other variations are possible): $5+19=7+17=11+13$.

2 POINTS

TOTAL 6 POINTS