Install this version:
emerge -a =sci-mathematics/calc-2.16.1.3
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask
emerge
autounmask =sci-mathematics/calc-2.16.1.3
Or alternatively:
emerge --autounmask-write -a =sci-mathematics/calc-2.16.1.3
calc-2.15.1.1.ebuild