Part property resold within 1 year after claiming exemption under section 54- income tax

Whether the property sold in 2017-18 was house property?

In that case, you were required to invest only Rs. 75 lakhs in a house property.

Which means only one flat is enough for capital gains exemption u/s. 54 of IT act.

Balance amount you can invest at your discretion.

Moreover there is no restriction over number of house property owned. (for claiming sec. 54 exemption).

Only point for objection is single agreement for all five flats....

Once that point is cleared, you are at liberty to sell remaining flats and pay tax over any gain, be it ST or LT.