Both ratios have a value for the number of JUNIORS.

So, to COMBINE the to ratios, we need a value for the number of juniors that's the same in both ratios.
To do so, we'll create EQUIVALENT ratios (by multiplying both parts of a ratio by the same value)

GIVEN:
sophomores : juniors = 2 : 3
juniors : seniors = 5 : 6

Take the TOP ratio and multiply both parts of the ratio by 5.
Take the BOTTOM ratio and multiply both parts of the ratio by 3.
We get:
sophomores : juniors = 10 : 15
juniors : seniors = 15 : 18

We can now COMBINE the ratios to get: sophomores : juniors : seniors = 10 : 15 : 18
10 + 15 + 18 = 43

So, out of every 43 players on the whole team, 10 players are sophomores.

Answer: 10/43

Cheers,
Brent