Question 234276
Let J = John's present age and S = Sally's present age.
1) {{{J = (3/4)S}}} "John's present age is three-fourths of Sally's present age."
2) {{{J+5 =(4/5)(S+5)}}} "In five years, John's age will be four-fifths of Sally's (not Sarah's) age at that time."
Substitute {{{J = (3/4)S}}} into equation 2) and solve for S.
2a) {{{(3/4)S+5 = (4/5)(S+5)}}} Multiply through by the LCD of 20 to clear the fractions.
2b) {{{15S+100 = 16(S+5)}}} 
2c) {{{15S+100 = 16S+80}}}
2d) {{{20 = S}}} and...
1a) {{{J = (3/4)(20)}}}
1b) {{{J = 15}}}
Sally's present age is 20 and John's present age is 15.