Question 721761
This relies on the search for whole numbers such that 20% gives a whole number and 1/7 gives a whole number.  70 fits this.  You can look in the multiples of 7, and choose from the ones between 50 and 100.  This is more effective than setting up equations {{{0.20(t+n)=t}}} and {{{(1/7)(t+n)=n}}} and use of {{{50<=t+n<=100}}}.