This can be reduced into the following system, where s is the amount of strong solution and w is the amount of weak solution:
s + w = 6.25
25%w + 75%s = 35% x 6.25 = 2.1875
Multiply the bottom by four to get rid of the percentages:
w + 3s = 8.75
Subtract the first equation from the second:
2s = 2.50
s = 1.25
w = 5.00