Пусть a - первое число, b - второе число, c - третье число. Тогда:
a + b + c = 21\\a = 2.5c\\b = 0.5c => a + b = 3c => a+ b+c=4c=21 => c = 5.25\\a = 2.5 * 5.25 = 13.125\\b = 5.25*0.5 = 2.625\\Answer:\\a = 13.125\\b = 2.625\\c = 5.25" alt="\frac{a+b+c}{3} = 7 => a + b + c = 21\\a = 2.5c\\b = 0.5c => a + b = 3c => a+ b+c=4c=21 => c = 5.25\\a = 2.5 * 5.25 = 13.125\\b = 5.25*0.5 = 2.625\\Answer:\\a = 13.125\\b = 2.625\\c = 5.25" align="absmiddle" class="latex-formula">