Let H be the number of heavy (50 kg) TV sets, and L be the number of lighter (30 kg) TV sets.

The total weight of all TV sets is 880 kg = (50 kg)*H + (30 kg)*L.

The total number of all TV sets is 20 = H + L.

We have 2 equations in 2 unknowns. This gives enough information to solve for H and L. Let's do it this way: Multiply the second equation by 30 and subtract it from the first. (Note we will drop the use of the kg units for this.)

(880) - 30(20) = (50H + 30L) - 30(H + L)

880 - 600 = 50H - 30H + 30L - 30L (use the distributive property)

280 = 20H (collect terms)

14 = H (divide both sides by 20)

14 sets weigh 50 kg.

_____

20 = 14 + L (put the value of H into the second equation)

6 = L (subtract 14 from both sides)

The total weight of all TV sets is 880 kg = (50 kg)*H + (30 kg)*L.

The total number of all TV sets is 20 = H + L.

We have 2 equations in 2 unknowns. This gives enough information to solve for H and L. Let's do it this way: Multiply the second equation by 30 and subtract it from the first. (Note we will drop the use of the kg units for this.)

(880) - 30(20) = (50H + 30L) - 30(H + L)

880 - 600 = 50H - 30H + 30L - 30L (use the distributive property)

280 = 20H (collect terms)

14 = H (divide both sides by 20)

14 sets weigh 50 kg.

_____

20 = 14 + L (put the value of H into the second equation)

6 = L (subtract 14 from both sides)