In May 2015, 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 2020, 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.