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)