The sum of 3 decimal numbers is 6. Exactly one of the numbers is less than 1 what could those numbers be.

There are infinitely many solutions to this problem (which is why it asks what could those numbers be), so you just have to make one up!

Start with the first sentence.

x + y + z = 6.0

Now one of those numbers has to be less than 1. I'll choose x. And I'll choose a random number, say, 0.7.

0.7 + y + z = 6.0

Now I'll subtract 0.7 from both sides to see what y and z have to add up to.

0.7 + y + z - 0.7 = 6.0 - 0.7

y + z = 5.3

Now I'll choose a random number for y that's less than 5.3.

2.6 + z = 5.3

And subtract 2.6 from both sides to find z.

2.6 + z - 2.6 = 5.3 - 2.6

z = 2.7

And now we have our numbers!

x = 0.7

y = 2.6

z = 2.7