if your 2016 taxable income before charitable contributions was high enough all of the $40,000 of contributions would have been used in that year. if for 2016 any was not deductible by reason of taxable income limitation, the leftover would have to be carried to 2017 and a determination made of whether it was all used up, if there was still a remainder unused, it could be carried to 2018. you need to carefully check 2016 and 2017 because if you missed deducting anu carryovers in earlier years, the IRS does not allow you to forgo amending prior years and using the 2016 carryover in 2018. Actually the IRS does not require you to amend 2017 but you must compute any carryover to 2018 as if the proper amount was deducted in 2017
