In May 2012, Ruth sold a freehold building which she had used exclusively for business purposes. The building was sold for £220,000, realising a chargeable gain of £42,500. In the following month, Ruth acquired fixed plant for £220,000 and elected to hold -over the gain on the freehold property against this plant and machinery. In August 2017, she acquired another freehold building for £205,000 and elected to transfer the held -over gain to this new building. Explain the treatment of the £42,500 gain.